Transmission - rough downshift

I have a 1999 Ford Windstar with 67K miles. At 65K miles I had the transmission replaced (non-dealer but lifetime warranty) due to low internal pressure and erratic shifting. A rebuilt transmission was installed. Van ran fine except that downshifting under load from 2nd to 1st produced a pretty serious "lag-then slam" type downshift. An error code revealed a problem in the torque converter. Van was brought back in, new TC installed, still not shifting correctly (so said the shop) and another rebuilt transmission and TC was installed. Within a few days the downshifting problem is back. The van shifts and runs fine except under that one condition.

It would appear one of two things is happening; 1) the shop (albeit a part of a national chain) doesn't know how to fix the problem or 2) the problem lies with a sensor outside of the transmission. Any advice would be most appreciated.

What type of ATF is in it. There is exactly one "correct" fluid, all others can cause this.

Mercon V fluid is the ONLY ATF that this transmission whould use. Standard "Mercon" fluid will often cause harsh shifting.

Not sure what ATF fluid the "national chain" put in. Will inquire. Thanks