I've had it with this 904, about to give up!

Maybe I can shed some light on this.
Sounds like a low pressure problem to me. (Soft or no engage and flaring shifts)

Did you go through the pump?
Did you chk the pump stator ring surfaces inside the forward drum?
New rings on the stator? What kind? (Steel, Teflon)
Did you chk the ring lands and inner surface of the governor ring bore?

The accumulator is not going to cause it to act like that unless it was outright missing. :-)

A REALLY good tell tale would be a mainline pressure test.