I'm able to, using my hands alone, wobble my crank arm about 1/8" to the left and right. I can only guess what this means for my riding performance, but I imagine it explains why when I pedal really hard my chain scrapes my front derailleur no matter how I fine tune it (it's non-indexed).
Does this mean that the bottom bracket isn't adjusted properly? Or, worse, that the BB is shot? My setup is about 12 years old.
If both arms move it is the BB. If it has been that way for a while you probably need a new one. If only the gear side crank arm moves maybe all you have to do is tighten that side. Does the crank us a cotter pin or 14or15 mm center bolt? Avoid riding it untill you can get it fixed.
Bike jocks ar still jocks
If it's a standard sized bike I would just buy a new BB, they really aren't that expensive, or just new BB bearings.