I have a C6 from a '72 mustang behind a 351C engine in my '70 coupe. Problem is the transmission has a good hard shift into reverse and also it goes into gear in drive when started cold. Once temp comes up to 190 and you place the shifter into park and then back to drive, nothing. Reverse always works. Rev it up and it just slips, nothing. Trans was rebuilt and I removed and cleaned the valvebody, worked for a bit but now is the same. Could this be dirt? Is there a way of working the dirt out of the trans in the car?