Take a look at this *extensive*, and incredibly informative, thread regarding slipping gears on the MPF:
http://pinside.com/pinball/forum/topic/new-pickup-dr-who-repair-thread
I had the same problem with slipping gears when the playfield was lowering, causing an audible "Ka-THUNK". After reading the above thread, I went the way of JB Kwik-Weld and it solved the problem.