Hard Shifts

99 Intrigue w/3.5 engine. The transmission shifted perfectly before a fluid change at 70k miles. My half brain dead mechanic left the trans. about 3 qts. short of fluid. I drove it that way about a day before discovering the shortage. I topped up with some "universal" atf sold at a convenience store. Ever since, the shifts, both up and down, are very hard, almost jarring, but only after the car has been in stop and go traffic for a while and is hot. Shifts fine when warming up and on the highway.

Since temperature seems a factor, you think its the no-name fluid I used? Should I try another fluid/filter change? Or is there some transmission part that acts up only when hot which may have been damaged when it was short fluid?
Thanks for any feedback.

defiantly change the fluid... and for the piece of mind change the filter again. Generally, the hard shifts are caused by the transmission going into maximum line pressure (fail safe) mode to prevent the slipping or delayed shifts it has detected. This will usually reset when you turn the car off and back on, but you said it keeps re-occurring so first thing would be to check the fluid and make sure its still bright red/pink and does not smell burnt from being low on fluid. If its even a bit dirty or smells off then change it. You need dexron 3 or dexron V fluid. There is an issue with these particular transmissions having bad EPC (pressure control) solenoids that cause slipping, and eventually the transmission goes into failsafe mode and sets the code DTC p1811 max adapt/long shift (causing very hard shifts, but eliminating all slippage). I would think this was your problem but seeing as it happened after you had the fluid changed and drove it while very low on fluid I would make sure it has the proper fluid in there and is not burnt. After this, get a transmission shop to diagnose with a transmission capable code scanner

also, make sure you did not overfill the fluid in the transmission. If too much fluid is added the fluid can become 'whipped up' and full of bubbles wich can cause slipping, which will eventually cause hard shifts when the computer detects enough slippage.

Thanks, harmankardon35. I'll get the fluid and filter changed the first chance I get.