PeteK wrote:
If properly set, the band (active for 2nd and 4th gear), is damn near bulletproof. I have a couple of friends making in excess of 600 hp and 500 ft lbs through a stock servo/band set up.
3-4 pack has always been as weak link, but having both fail together is rare.
I would lean towards a line pressure problem, or a broken sprag in the converter.

Either way. Stop driving it. every time you whack the throttle, you may be tearing up a part of an otherwise decent core.