Can't seem to get a consistent answer from you - previously you mentioned when you pulled the converter it had plenty of fluid. Now you say it had little fluid the last "few times pulled". Which one is it ?

A partially filled converter will get you EXACTLY the issue you have. You did not reply as to if you checked to see if the perimeter balls were in place on the support. You can have normal line pressures but if the converter circuit is open you will get a slipping feeling. One of the balls on the perimeter is for the converter charge circuit.

Also you say you have good cooler flow - but have you measured it? With a normal 3/8" line disconnected at the return - rear fitting at the trans you should get at least a quart of fluid at idle in about 10-12 seconds.