I was riding my friends 05 Hardrock Sport today, which I may be buying, and came across a problem. I was cruising down a fairly steep hill and bunny hopped off of the sidewalk and into the street at speed. A few minutes later I noticed that while pedaling the left pedal's travel felt out of round. At first I thought I'd bent the pedals shaft but upon closer examination I realized the left crank arm bolt was very loose and the crank arm itself was wobbling back and forth. Not having a toolkit on me I hand tightened the crank bolt and continued on my way... for about 20 feet, at which point I felt the same out of round feeling as before. I rode home, stopping every few minutes to tighten up the bolt so as to minimize any potential damage from riding the bike this way.

After I got home I was able to tighten the bolt, but it still seems to want to back itself out almost immediately after pressure is applied to the pedal, and the crank wobble returns.

I'm just wondering what the likely cause of this problem is, and how it can be best resolved.