I don't see the valve body having anything to do with your description unless (highly improbable) a valve is somehow hanging and then breaking loose in a repetitive fashion. They just don't do that even when really trashy internally.
Having said that...if someone really screwed around with the "adjustability" of the few control points on the valve body, then that is a different story. I'm dealing with one like that now where someone tried to make it act like it had a"shift kit" by simply cranking on the adjustments which are totally screwed!
A torque converter does not control or "put out" pressure. The oil pressure is produced by the oil pump mounted in the front end of the transmission and oil pressure/flow is directed to various operational components by the valve body and it's feedback loops. The only variable in a non-lockup type torque converter is it's assembled stall speed. And since you have mentioned this is a van, then those would have never had a dam lock-up converter pos! The lock-up stuff was only available in the light duty/light weight vehicles, their durability sux.
I have no issues in helping anyone with torqueflite stuff...I first learned this stuff the hard way as a mechanic-trainee in a chrysler dealership in 1968 and then taught this stuff in an automotive technology program in a community college for 14 years. A 904/727 is a 904/727, we ain't prejudiced!
When the replacement valve body was installed, are you sure that the spring was installed between the valve body and the accumulator servo/piston?? Most of the "shift kits" sold out there call for eliminating that spring if the kit is installed along with all called-for mods. But with a stock valve body/trans you cannot do that.
The other thing that comes to mind...even though a "pro" May have built the trans...is that the end plays in some of the critical areas are excessive. End plays are set according to the specs (which are really a pretty large tolerance) through the use of selective thickness shims, both metallic and fiber.
Another thing that comes to mind...iirc, a van used a "slip yoke" on the forward end of the driveshaft. With the rig up on stands and the trans in park, describe the "slack" found at that point, you could have an excessively worn slip yoke. If I'm wrong and it has a "bolt-on" yoke like the motorhome version, then double check the torque on the retaining nut.
