T 3650 common problem
It's the crappy tremec 3650. My car won't go into 3rd if I try to shift above 5k RPM's. If i shift it slowly ... it will go in, but shifting slowly causes the engine to drop below the 5,000 RPM range. A lot of people have this problem. Check out Tremec 3650 issue ... threads. I had this problem with stock shifter, and Steeda Tri-ax. We should count our blessings, cuz this is what the better T 3650 trannys do!