Failure in Transmission Thunderbird SC 1994

Hi, I'm new to this page and first of all, congratulations on how useful it is.

Now, my problem is the following: I have a 1994 Ford Thunderbird SC which has had a transmission failure for some time.

It started with a problem when braking at a stop sign. When trying to accelerate again, the car felt very sluggish (like it was idling) and the gears would suddenly shift very roughly. It's worth mentioning that the problem only occurred in this situation. When completely stopped and the park lever was shifted to any other position, there was no jolt or anything like that, nor when accelerating; the gears shifted smoothly and on time. After this problem persisted, it began to affect the other gears, for example, from first to second and so on. The gears would shift very roughly even without braking and while continuously accelerating.

Later it went into a workshop for a major transmission repair, they changed the clutch plates, turbine and other parts that suffered wear and breakage due to the abrupt changes.
When it came out of the workshop, the transmission felt great and there was no problem under any circumstances, but after a month it started to do exactly the same fault as before it was repaired.
The car was returned to the shop under warranty, and they said they cleaned the valve body because it was very dirty. It was supposed to work perfectly again, but this has happened four times now, with the same diagnosis: a dirty valve body. The problem only occurs when braking at speed bumps and now when reversing; there are no issues with shifting gears under constant acceleration.

My question is about what could be causing the valve body to get dirty, as the workshop specialist mentioned, or what other cause it could be.

The only other engine-related faults I know of, which were already fixed before the failure and haven't occurred simultaneously, are an error code in the IAC valve, the TPS, and recently the EGR.

The IAC valve issue was fixed, leaving only the EGR and TPS issues.