Changing the number of teeth on your ring(s) can cause problems as the shifting ramps/notches are positioned for certain 34X50, 36X50, 42X52, 39X53. etc. if you chnage one you have to get it to match the other size, and change the other size so the ramps match the new replacement. The chanrings need to be oriented to the crankarms rotationallly in a specific way, in most case a protrusion on the inside of the ringm will need to be aligned with the crankarm. Some inner rings have a slight offset to the teeth some don't, changing ring offsets can cause problems as well. For a chain that drops off, sometimes the only "fix" is a "chain keeper", ther are several designs see- http://www.amazon.com/Redline-Cyclo-...e+chain+keeper for simple and cheap option- most shops stock at least one type