slip

Ok I have an issue I need help solving, all guys with 2.9L/3.4L whipples feel free to chime in. My car was in the process of getting tuned and has massive belt slip after 5300-5400 rpm. car noses off from 26.5psi at 5400 to 14psi. (Made 690whp at that point, so much more to be had :banana:)...