Bicycle Tutor

Hi,
I have a threadless stem on a Specialized mountain bike. I've noticed that after a few weeks of riding my front tire is no longer 90 degrees with my handlebars. It always shifts toward the left if you are sitting on the bike. The handlebars themselves are not shifting on the stem so I think it might be the stem that is slowly shifting along with the fork. Any ideas? Everything has been snugged up but it continues...
Thanks!

You are right that it is the stem that is shifting. The stem is actually clamped directly to the fork where it sticks up through the top of the frame and is slowly twisting on the fork. Here are a couple things to check:
1. the fork should have grease on it where the stem clamps on, but not too much. Just a very thin coat to prevent corrosion.
2. The bolts on the stem should be well greased so that they can be tightened down smoothly.
3. make sure there is still a gap on the stem where the bolts tighten. If it tightens down so much that the two sides close the gap completely, there is something wrong with the stem.
4. check the area around the clamp bolts to make sure there are no cracks or anything bent out of place. This would be a sign of imminent failure and big danger.
5. Since the stem also controls the adjustment on your headset, you should be checking for looseness (play) regularly. If the stem is slipping, the headset may also be loosening up which can damage the bearings.
I'm not sure why your stem always goes one direction. If you have a suspension fork, you may want to have both sides checked to make sure they are working evenly. If one side had more pressure than the other, it could cause the bike to pitch a little when you hit bumps and that may cause the bars to rotate. It also may be that you tend to break more with your right hand, thus keeping your right arm tense, thus putting more shocks to that side of the bars.

Thanks for the detailed response! I'll play around with it and report back~