Hello Everyone!

I just purchased a 2014 Chevy Silverado WT with 190K, the 4.3L V6, 6 speed trans, and RWD. On my way home, I lost what seemed to be 5th and 6th gear and popped a check engine light. The temp outside was probably around 70 degrees. The truck's transmission would seem to act normally for a short time after being shut off. My wife and I let it sit for an extended period of time and drove home later in the day. On the way, I stopped at an Advanced Auto Parts and ran codes, finding I had a TCC solenoid code. I was also about a half a quart low on transmission fluid. I added some Dextron VI and the truck drove normally all the way home. The temperature outside had cooled significantly. The next day drove it around town and found that if I didn't drive on the highway, I could drive all day with no issues. I sent the truck to my mechanic and while it was with them, it had no issues shifting at speeds from 30mph up to 80mph on extended drives. The check engine light went off on its own. These were also cooler days. They did note that transmission temperatures reached 240 degrees. Picking it up and driving it home today, I found trans temps reached 225. I've been told that I should swap out the transmission with a low mileage salvage yard unit. Before doing so, I wanted to make sure that was the right direction to take. If I needed a replacement, wouldn't it not shift properly all the time? Could higher temps cause the TCC solenoid not to operate? I have seen that there is a thermostat for the transmission. Couldn't this be a defective thermostat causing the issues? Any insight is appreciated!