Normally pretty good boxes, so long as you don't overheat them.
They of course do have their problems though, first thing to do when losing drive, is check the fluid level, but I think this is pointless in your case.
It sounds VERY much like your forward clutch drum has gone, with the constant movement the circlip that holds it wears a groove in the drum.
They also suffer from overpressure problems, very commonly, but there is a modified valve chest and torque convertor that cures this.
The biggy is the cost, you are looking around £800 - £1000 for complete strip and rebuild to modded spec.
tabetha