Bicycle Tutor

Hey!

My bike has 3 "big gears" and 7 "small gears". At gear 2x6+7 and 3x6+7 I've encounter problems. When I try to tread too hard, it hops on and off the gear so that it makes it impossible to use those gears in anywhere else than downhill when I don't have to tread so hard.

I've tried to adjust the gears, but it doesn't seem to change anything. What should I do?

I don't really know what gear combinations you are talking about, but a chain skipping under load is a sign of chain wear. Check for it. Also note that a heavily worn chain may have worn down both the sprockets (in the rear) and the chain rings in the front. At least sprockets and chain probably need to be replaced.

The one Joe mentioned is the most likely; and you need to replace your chain and freewheel (likely you have a freewheel and not a cassette).

Less likely; one or more stiff links in the chain.

Given that the issue occurs in both the middle and large chain ring, and with both the smallest and next to smallest sprocket (cog); it is very unlikely that it is a derailleur issue.

aha.. Thanks for the answers, though they do not lighten my heart.. It would be nice if I could fix this my self, but it seems I might have to buy either a new chain and/or freewheel(s?).. I'll do a checkup on them both to see if they have wear..

Well, replacing the chain requires a chain tool. Every cyclist should have one anyway.
Then, you need to check whether you have a cassette or a freewheel http://sheldonbrown.com/k7.html and get the correct replacement part. While it would make sense (in my opinion) to buy a cassette tool (well, if this is what you have). A freewheel tool: not so much, as there are about a bazillion different types out there and they are dying out.
Replacing the freewheel or cassette takes about... 5 minutes. Replacing the chain about the same. It is not rocket science, but as with all things: read about it first, go through the steps, follow instructions carefully if you never did it before.