What makes the chain fall off?

[drplasma64]

Yeah, something is out of whack for sure. The chain had only fallen off once since I started riding again, and then all of a sudden it's having huge problems. All the components are the originals...(except for the chain) shimano SIS derailleurs, biopace crank.

It really has gotten bad since my first ride after I changed my tires. Tonight I just rode on the small chainring after the first time the chain fell off (a block from my house). I had two ideas:

1. My frame pump fell off on that first ride with the new tires...could it have hit the FD and put it out of alignment?

2. I read that the RD has to be aligned first before adjusting the FD...is it possible that when I put the back wheel back on, the RD is no longer where it should be (relative to the freewheel), and so that's affecting the front shifting?

Or could it just be that cables are stretching? I had a tune-up about 60 miles ago.

Also, I checked that park website...it sure looks like the FD is WAY too high to me, but that isn't something that has changed recently.

Sorry, I guess this should have gone into the maintenance forum. And I'm definitely going to sign up for a bike maintenance course!