Jesse I would not worry about the cooler right away. I initially for the scan found a tranny shop that scanned mine for free and yup I had the dreaded P1811 etc.
Before I installed the TransGo shift kit I added this to transmission fluid run for 700 miles full strength, full bottle and drove normally. That cleaned things up great, removed varnish and only once or twice encountered a milder harsh up shift.
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Ok so then I did install the TransGo shift kit and it was clean as a whistle in accumulator!
I Installed it in half hour including scuff the bores with trans fluid wet 1000 grit emory cloth. The transgo kit came with good instructions that makes ease/sense once you have it apart.
Had about 6 quarts ready to refill while cold and two more on standby if needed to add slowly when hot, after driving, to then reach the fill line on Dipstick in my 97Pa I think it took 7 quarts total.
Used the latest syn DEXRON VI
(6) fluid !!! (that fluid is awesome hardly even gets hot after highway drives)
I have not had one harsh upshift since. I also have new saprk plugs, wires etc so the engine runs perfect and no other codes.
Get the kit in one car with new filter /fluid see how that works out I am sure you will find it much better. Replacing the pressure solenoid was not an option I wanted to discover, and I cheated it now.