Yep running a new chain on an old cog can be very hairy. The cog will have "adapted" iteslf to an old stretched out chain, but once you put the new one on all kinds of funny stuff will happen. Recently i replaced the chain on my geared bike and the chain would slip off the chainring in front when I really torqued on the cranks, basically at the worst possible times. So I replaced the chainrings and had no problems after that. Ultimately, the solution to this problem is to replace chains more often so that stretch can't cause rapid sprocket wear.