The EPC is the main pressure control, and critical for the 5R and newer automatics. The computer has full control of the pressure with the EPC, so you can't have that one getting old and worn. The early 5R55E EPC was super high, $125 as I recall, but now they've come down to $75 or so I think.
I'd do that EPC for sure, and if you can it soon, the Sonnax pressure regulator best for your trans. There are several individual Sonnax parts and kits for most common transmissions. I like their VB o-ring kits and the PR for helping the performance, plus the solenoids you think it needs based on mileage.
The VB kits are great of course, the aftermarket has just come up with more/other little thing to improve things. Drive it easy on the shifts, let off just an instant before it shifts, then you can use more throttle in the next gear, until the next shift. I notice any poor shifting usually when the trans is cold, like the mornings we have now. My 95 Crown Vic has a little weakness in its two first gear shifts, and I don't push it right when it's going to shift(I let off a bit until it shifts).