Originally Posted by
Homebrew01
Are these downtube friction shifters ? Sounds like 2 problems:
1) Your shifter is loose, allowing it to shift from the rear derailleur spring pulling on the cable. If downtube shifters, tighten the little wingnut or screw on the side.
2) If the wheel is pulling then A) Your QR lever is not tight enough. B) You have the less powerful exposed cam QR lever. 3) You could use a more serrated or "grippy" outer locknut on the drive side. 4) All of the above.