700r4 rebuilt with Monster Kit. Bigger servos and bands. Superior shift kit. Running a 400 sbc about 375 hp. In a 1934 Chevy street rod (2900 lbs) with a 2000 stall convertor. Shifts at 3000rpm in all gears. Doesn't slip or make any strange noise. Attached pressure gauge to transmission and it has 175 lbs in overdrive with the convertor locked up. Everything fine until the weather gets hot. (above 60deg.) When out side temp goes above 60degs I cant get to shift into overdrive. Running new steel lines thru radiator and then thru external oil cooler. I can hold my hand on return line and its not terriable hot. When the out side temp gets above 60degs, the trans pressure gets below 150lbs is when it will not shift into overdrive. Have had the pan off and checked TV cable too make sure it is fully depressed with a wide open carb throttle. There is no metal or crap in the pan and the trans oil is red and does'nt smell. Just running Carquest Dex/Merc trans oil. Should I use a different oil? When trans will not go into overdrive it stays in drive running 3200 rpm at 75 miles/hr. Doesn't slip, going down a hill when I let off the gas it seems too be in neutral until I bring up the rpm's and then it seems like I put it back in gear. Any suggestions?