A friend of mine has a issue that just developed. SBC with PG & 8" ATI Converter. Car leaves fine & eveything is normal through low gear.Once high gear shift the car feels like something is dragging it. Slow to build RPM & car is approx 2-3 tenths slow. Fuel curve shows rich condition, Crank trigger pickup ohms ok. Transmission was tore down & all parts looked ok. Converter was sent to ATI. They said it looked ok. Any ideas

Tire rub? Drive shaft rub?

I would say the sprag is going away, even if ATI wouldn't admit it. I had the same thing happen to me and was even intermittent until finally crapping out completely. That is my guess anyway.