There are 2 valves that time the band release and the clutch apply. They take throttle pressure against governor and spring pressure to give shift quality (cushion it under light throttle conditions).
With your engine modifications, you are putting a lot more torque at less throttle, your trans thinks your cruzing so it shifts accordingly.
I'm not an expert but I highly suggest you call who ever rebuilt it or get someone who can take pressure etc... and fix this slip before you fry something.
I had a leaking servo cap and fried my forward clutch and band in one evening.
The only thing I know would be to check pressures.
Usually when problems gets worse with heat, its a leak in a circuit some where.
Sometimes a rolled piston seal won't be a problem right away. I've seen it in clutch packs in transmissions and piston seals in hydraulic cylinders; rolled seal, works fine, out of nowhere problem.
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