Originally Posted by sivat
If you're "mashing" up a hill, you're probably swinging the bike from side to side. If timed just wrong, the chain will swing to the side of the chainring just as the next tooth is moving into contact. Once one tooth is outside the chain, the rest will follow. This is probably exacerbated by the frame flexing. Try to get the chain a little tighter, and make sure your chainline is ok. The further out of line the chainring is, the more likely it is to happen.
I love this forum! After reading all the suggestions and a more detailed review, while chainline looked ok, the chain
was slightly slack. Slack enough to take advantage of Sivat's description above. So, a little loose chain and "mashing" (well almost mashing!) and boom.
I moved the wheel back enough to increase the tension and tightened it up real good.
Thanks to all!
BTW: frame is small - 52.