Could a bad thermostat be the issue?
The only time I ever had it go into limp mode, it needed new shift solenoids. I just had it towed and I think it cost around $300 for the repair some years ago. Not saying thats what it is but just one possibility. I’d just get it into the shop and not keep driving it.
Have you checked the TV (Throttle Valve) cable that controls the shift points on a 700R4. The TV cable on my 93 Caprice was out of adjustment and was causing delayed and violent shifts. Once the cable was readjusted the trans shifted normally. A fraction of an inch can cause big changes in shifting. Here is a link to an article that can explain it better. Google “TV cable adjustment” for more sites.
http://www.cpttransmission.com/tech_tvcable.htm
Good luck,
Ed B.