First order of business would be dropping the pan, if it's full of clutch material and metal glitter it'll need to come out.
When you first pulled away from the light and it was slipping, was that in manual Low ("1") or in Drive? Does it still slip badly in first if you have it in manual Low?
When it was rebuilt, were any upgrades done to the hard parts or valve body or was it just rebuilt with stock parts? 904s won't hold up very long to spirited driving without at least a shift kit to increase hydraulic pressures and make the shifts quicker.