That's where tuning the fans to turn on at thermostat temps helps. Stock thermostat is 195F, the fan turn on temps are somewhere around 216F for low and 228F for high speeds. As you can see the engine temp is going to be up there if you are in traffic. The trans is cooled by going through a small cooler that is in the side of the radiator so... your coolant is either cooling or heating the trans fluid.
Many 4T65's tend to run warmer than we like no matter what. Others can run nice and cool. It is possible to add trans coolers to them. If you want a factory option look up the impala police package trans cooler and lines. Otherwise most folks might add an aftermarket cooler. But we are all a bit nuts anyhow. If you are getting the bang or slam shifts as you mention that is likely accompanied by the P1811 code as mentioned earlier. This means the pressure control solenoid is being commanded to max pressure due to longer than expected shifts happening. Which makes the exact opposite occur. BANG!!! Easy way to get around it is the transgo shift kit (also mentioned by someone). It isn't the true fix to the problem, but it does keep shifts faster and therefore more firm w/o the max pressure adapt occuring. So it fixes it w/o a transmission rebuild.
Me.. I ran a stock GM cooler because my 97 Bonnie was the same basic body as the 92-99 that had the cooler on the SSEi's. Older caddies and Park Aves also had these coolers. I mention this in case you want to go the cheaper junkyard way. Does it show I used to scavenge stock parts at the yards?
