Transmission Fully Rebuilt Still Experiencing 3-4 freewheeling slip when warmed up

[bhef 0]

I recently rebuilt the 4l60e on my 2000 S10 4.3L 2WD.

It was slipping out of 4th while in D so I drove it for a week in 3. Eventually, 3rd gear was slipping, especially when warmed up, so i decided to rebuild.

I got a vacuum tested reman valve body from Shift Rite Transmission shop with the Sonnax TCC Isolator Sleeve installed. It included all new solenoids. I also got a reman pump (old pump was scored up inside) from same shop with upgraded Sonnax boost valve. I installed pinless accumulators and new Transgo seperator plate with new gaskets and new checkballs. I upgraded to Sonnax smart shell and installed Sonnax Heavy Duty 3-4 backing plate with appropriate steels and frictions. All new steels and all new HE frictions - All clearances (including input shaft endplay) within spec checked 3+ times. New Borg Warner Dual Input Sprag. New reverse input drum. New 2-4 band. New 3rd accumulator checkball capsule tested for leaks. Sonnax Servo Release Check Valve installed over it. Standard Toledo rebuild kit for seals and gaskets, etc.. Also, new VSS and new Neutral Safety Switch. All new bushings. 400 grit emery cloth sanded journals on everything. Cleaned MAF sensor with cleaner.

I diagnosed a leaking input drum housing at the shaft and got a new one with new blue rubber pistons. Old 3-4 clutch pack was in decent shape from before but still replaced new. Air-tested new input drum cold and it did not leak where the shaft and housing meet. The plate stiffener was loose during disassembly so after rebuild delayed thud into Reverse was fixed.

When I bought the truck about 500 miles before the 3-4 went out the CLUSTR fuse was blown causing limp mode. After replacing the fuse truck drove fine until 3-4 problem. This leads me to believe that there is maybe some electrical issue, but no code was thrown until the slipping got terrible. It was solenoid A electrical (P0748), which I replaced and code disappeared. It came on again a few drives later and then disappeared the drive after that no changes on my part.

During test drive after rebuild it was SO damn nice and smooth at first I couldn't believe it. THEN after it warmed up all of the 3-4 slipping returned and I had to drive it home in 2nd.
I disconnected the MAF sensor and drove it until warm to rule that out. No luck there. Slightly firmer shifts, but did not help slipping.

I'm thinking now that possible causes could be:

1. New solenoids - one may be bad,
2. The input drum doesn't leak when cold, but does when hot,
3. The 3rd accumulator check ball is bad when hot (I wasn't too sure of my vacuum test being accurate, which said it didn't leak, so I tested with 50/50 solvent and ATF and it did not leak while cold),
4. or there is a bad wire somewhere considering the original P0748 code that came on and off 2 times.

ANY insight or ideas would are very appreciated. I've spent countless hours on this and is my 1st rebuild.