I have had this problem and think it was due to my belt being a little old and hard. The belt was almost new but I think it sat on the shelve and in heat for a long time before I bought it. You might also try a different brand. I have had good results with Gatorback.
98 Tan on Laser Red Rag Top Cobra
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Fully built and forged with Mahle pistons, SHM Blower cams, ported heads/nitrated valves
Novi 2000, 52# RC injectors, custom IC, Devilsown alky injection, SSR Intake, Steeda rails and -8 feed lines, 03 tank + Aviator dual pumps
Tuned with SCT Pro Racer Package (and everything else over the years)
Lot's of other stuff!