I had the same issue at a race last year,everything looked good, but kept throwing belts. The lower pulley ,factory single belt had cracked at the mounting bolts.Could not notice any wabble,so I go out for a first round ,dial 11.10 to his 9.90. As I shift into 2nd the car made an inmediate left turn heading straight for the guardrail,luckily I steered away and did a figure eight on two wheels and barely missed the opposite wall,The good news the other guy missed inches tee boning me.The pulley broke off the bolts cut the radiator hose so clean it looked surgical. Long story but please check your stamped pulleys for cracks.I will no longer use a stamped pulley on anything race.