Interesting!
It could very well be the belt slipping on the pulley on the engine or, the clutch. What diameter upper pulley are you running? I have one on my "parade tractor" that is about 7 inches and it speeds up the little cub pretty fast. It also slips once in a while when I first start out. it's an aluminum pulley but not like the original one.
My guess would be, if nothing else has moved around, that it most likely is a belt. Did it stretch a little bit from use? Maybe a little bit of tightening per the instruction manual might help it out?
I've got a 10-horse in one of mine and it has the original-sized pulley on it. It's actually our pulling tractor. I'll include a picture of it. Interestingly, the belt doesn't slip on it, even when pulling. It has enough grip to stall our 10-horse with the nose in the air!
An original with a creeper and lights, that's pretty cool! Here's ours.
