I had similar problems with my 105 triple. I eliminated the chain
falling off the smallest chainring with the NGear Jump Stop, $10. Here
is a review of the product.

http://www.mtbreview.com/reviews/Cha...ct_20650.shtml

For further information see:

http://www.gvtc.com/~ngear/whatis.html

I've been using the NGear for more than two years now and have not had a single chain drop. Works great!