Scour Up your Sheaves
I just spent all weekend trouble shooting a similar problem. Believe it or not but cleaning/scouring up BOTH sheaves and tighting up the belt (less deflection) did the trick. Works like a top now. I could not beleive the effect of smooth sheaves did (guess I waited too long to do it).
I use brillo, then got best results with 200 grit. Make sure you clean the dust off real good (dry wipe, then warm soapy water and dry wipe again).
Best belt deflection is when the belt is just barely squeaking at standstill - don't go by the book (measurement) if you want it spot on.
My theory is the sled (m1000 09) has so much power that if the sheaves are smooth the belt will slip. I remember going all season with lower power sleds without scouring/cleaning (when I was younger & dumber