Bicycle Tutor

I have a problem with a bicycle I built recently. About every two weeks or so, I notice a wobble in my wheel and notice there is at least one spoke that is lose. And considerably lose. So, I have tightened it and trued the wheel. Two weeks later, same issue. I am going to start marking the spokes from now on to make sure if it is not the same one(s) that are the problem. Perhaps just replacing those spokes may make the most sense.
But, the question is this: Is there a potential reason this is happening barring just a couple of bad spokes. They are used wheels and are basic alloy (700c) rims. When I bought them, they seemed to spin fairly true and I see no damage on the rim. I am just wondering if anyone has seen this before. And, if so how did you fix it.
Any help is appreciated.

If the overall spoke tension is low, that can allow individual spokes to loosen up. A good bike shop should have a tension gauge and can test the spokes for you. It's hard to be accurate on this without a gauge.
If the rim itself is bent and you have to have uneven spoke tension to get the wheel straight, that can also leave loose spokes which will unscrew themselves. Not too much you can do about that except the next suggestion.
It is curing the symptom rahter than the cause, but you can put threadlock on the spoke to prevent it from unscrewing. Use the non-permanent stuff so the nipple can still be turned (you don't want to 'glue' the spoke in one position). But that may help if vibration is jsut caused the nipple to loosen.

I have been experiencing this same issue on my new commuting bike. Yesterday one of the spokes actually broke at the hub.
Took it to my LBS today to get new spoke & have it trued & had an interesting conversation with the mechanic.
Acording to him this phenomenon is nothing too new. They see it a lot on wheels at the cheaper end of the spectrum - ones that are machine made coming out of China. I don't think i can recall his exact explanation word for word but this is my interpretation of what he told me.
He says it to do with the way the rims are "laced" on machines & only seems to affect disc brake rear rims. Apparently spoke pairs are not laced on the same side of the flange but on opposite sides - due to the machine assembly. He showed me on my wheel how one spoke of a pair (don't know what really determins a "pair") was laced on the outside of the flange while it's opposite was laced on the inside of the opposite flange. Clear as mud right!
He went on to explain that the added torsion placed on rear wheels with disc brakes can cause the spokes to come loose over time. The only way to correct this once & for all would be to have the wheel rebuilt - for a cost of $50 aussie dollars. Perhaps he is just BSing me.
I am still doing reasearch on this.

It's an interesting theory. And it is true that disk brakes torque a wheel differently than rim brakes. But I've seen wheels do this without hub brakes. I think the conventional wisdom amoung wheel builders has generally been that minor differences in lacing don't matter much. There are some who claim there is a difference, but no one seems to be able to show any data to prove it.
However, what pretty much everyone agrees on is that spoke tension is critical. Machine built wheels tend to hand slightly lower tension than a well done hand built wheel. Nickh's mechanic probably has cured the problem by rebuilding the wheel. But I'm not convinced that it was fixed by changing the lacing pattern. I think you just end up with a well done hand built wheel.
But, alas, I also have no data to prove this...

Just wanted to follow up with this as I may have remedied the situation and someone else may be able to try it as well to see if it works. I noticed the problem especially when I went on long rides and could feel the wobble and would feel very lose spokes.
I noticed I never had this problem with the front wheel. Now, that makes sense since the dishing is different, and there is a lot less weight and stress on it. But, I felt the tension of the spokes on the front and compared to the ones in the back and there was noticeable difference in how much tighter the front ones were.
So, when I found a lose spoke (and I did keep track and they were different ones most of the time) I decided to get the wheel trued. Once I got it to where it was fairly true, I located the stem and did a quarter turn on the non-drive and half-turn on the drive. I then felt and compared the tension the front and it was still a little different. I went around and did the same thing again and the tension seemed to match. I simply just touch trued as it did throw it off a little and did the other things in the tutorial and it seems to be working fine now. I did this maybe two weeks ago and, I ride every day at least 5 miles. The real test was the 26 miler I did yesterday and seems to have stayed true.
So, in a nutshell, just make sure your spokes are not too lose. If they are, give them a little tightening without over tightening.

Glad you seem to have found a fix. One note for others out there reading this thread. On a rear wheel with multiple gears, the spokes on the drive side will have higher tension than the non-drive side. This is due to the "dish" on the wheel which pushes the hub itself off center to allow space for the gear cluster. Normally you get the right hand spokes set to the "proper" tension and just live with the fact that the left side are a little looser. So in Buzz's case, you would match the right side rear spokes tension to that of the front's spokes.
Note also, it's hard to measure tension accurately by hand. Ask a shop to throw their tension gauge on it. Should just take a minute to see if you're in the ballpark.