Chainsaw Repair

Poulan Pro PP4218A
I keep the chain sharp  and correctly tensioned. I haven't lost the adjusting screw, but I see how it can happen. Every once in a while I'll look down and the adjusting screw is spinning. The chain doesn't loosen and the bar nuts are tight to the clutch cover.  When I take off the cover I can tell that the pin on the adjusting mechanism has disengaged from the bar. I don't see any damage anywhere, there's no obstruction, and neither the bar holes nor the pin appear to be worn.  Any idea for why this occurs and how I can prevent it? Thanks.

No, it's the "screwdriver" type. But I think I may have solved the problem. 
Once the chain is properly tensioned, and the clutch cover nuts are tight, the bar doesn't move, and the chain doesn't loosen. BUT - there's no longer any stress on the adjusting pin, and the vibrations of the saw shake it loose from the bar hole, and spin the screw.
So yesterday, after getting everything set, I gave the adjusting screw an extra ½ turn, forcing it against the bar hole, but not moving the bar (because the nuts were tight). So far the adjusting screw hasn't spun! I'll keep you posted. Thanks for the reply.