Have you verified that the fluid is full? Fully hot trans, in park, running and on level ground.

Is it the rpms bouncing up and down when you try to accelerate w/o downshifting?

Beyond that if you're going in again I'd add a trans-go shift improvement kit to it. It won't make it bang the gears just quickens the shifts to minimize clutch wear. I have one in my GT with around 280K on it and it shifts just fine.