There could be several culprits:
- Check head set tightness.
- Check the tightness of the front wheel bearings. This one got me for a long time before I figured out what was happening.
- If it is a large frame,e.g., 60cm or larger it might be inherent in the frame. When the oscillation starts press your knee against the top tube as a damper.
- Check the "trueness" of your wheels.
Good luck.
Last edited by Doug64; 10-22-12 at 10:57 PM.