MX-3 AT down shifts at high speed - hold light flashes

[MustStangSally]

1992 MX-3
Auto-tranny
4cyl
90K

The hold light periodically flashes but does not normally cause issue. Every now and then though, especially when around highway speeds, the RPM races and we can't drive much faster than 50mph. I checked the trouble codes and found:

1) 25 - Solenoid valve (pressure regulator)
2) 62 - 3-4 shift solenoid valve

I heard there might be a service bulletin covering TC 62 but cannot find anything.

I'm assuming these are located in the tranny somewhere around the valve body. The Haynes manual is next to useless. How difficult to replace? Can these be cleaned? My fluid color is a transluscent pinkish red. It's really quite hard to see how much fluid I have but it appears overfilled a bit. Could an overfilled transmission cause this?