5R55 faults after very late filter/fluid change 1998 300k

[UgLeeXLT]

Brief History:
Very poor maintenance history.
Hesitation in changes and slow to engage reverse when cold.
Young driver for past 10 months.
Car was given free hopefully for my son to get comfortable driving in.
I dropped the trans fluid and replaced the filter.
No leaks from pan.
Total 6 quarts replaced Mercron V Napa brand.
Level now reads at top of hashed area on dipstick.

Current faults:
When warm will only engage D after selecting 1 or 2 on gearlever.
Sometimes won't engage R.
Won't re-shift to R unless engine off and brief pause before restart and then R engages.
In D whilst driving normally it will sometimes rev as if manual clutch had been 'slipped'.
Occasionally won't pull away from static and feels as if a manual trans 'clutch' is slipping.

I've just redone my wifes 4.0 mustang SOHC engine timing chains and was hoping this was just a simple fluid change but seems to have caused more problems than it fixed...

 

[CDW6212R]

With that mileage, and the younger driver, guess of poor maintenance, I'd bet it needs all solenoids at a minimum, with new VB gaskets and accumulators. The fluid change did not hurt anything, something was amiss already, no matter what symptoms were noticed afterwords.

These 5R's need the EPC solenoid changed every so often say 100k miles or ten years. The other solenoids are not as critical, but to change the EPC, you are there and the others only cost about $60-$75 IIRC.

BTW, the original EPC solenoids were a lesser design, there was a test program that we participated in here, to test a new Borg & Warner EPC. Those became the new standard, so be sure any new EPC is that new type. I'd guess the old ones went obsolete ages ago, the program was done around 2002.