It could be several things then. Slipping torque converter, worn out clutches, bad pump, internal hydraulic leaks, a lot of things. But there is hope. A GOOD transmission shop can hook up gauges and see what the line pressures are. There is a plugged port on the side that a pressure gauge screws into and they can test for line pressure. A T-400 rebuild is not cheap but if you go for the best quality parts it will probably outlive you. A cheaper T-350 will bolt in but needs a different cross member and drive shaft. With a Pontiac 400 I would re-build the T-400. I once re-built a T-400 in my garage with basic tools. It was difficult but not impossible. My mistake was adding a shift kit that made the tires chirp every time it shifted. Lots of fun, but I forget that by boosting the line pressure, it needed an extended warm-up time in winter. So one day, running late for work, I blasted off only to hear destruction going on in the tranny. Live and learn.
