Turns out it's all of them.
I rigged up my 3PH snow blower this past weekend and went throught he usual lube and inspection process and while running the blower and dripping oil on the drive chain I noticed the chain jumping every time around. I shut things down and found a link coming out.
Closer inspection of the chain reveiled about 75% of the links had fractures and failure was immenent. Needless to say I'm off the buy a new chain, but I think this speaks for the benefits of a slip clutch over a shear bolt. If only I could get the drive shaft off the gearbox shaft, I'm be able to add a clutch.
Check your drive chain - it may be the weakest link.