it's possible the stator clutch in the torque converter is not freewheeling. it may be seized up. the only way to know for sure though is to take out the converter and try and turn the clutch by hand.
YOUTHINK THATS STRANGE THE FIRST Z I HAD WAS AN 80 280ZX AND FOR FIVE DAYS IT WOULD NOT GO OVER 35 MPH I COULDNT FIND ANYTHING WRONG WITH THE CAR THEN IT JUST FINNALLY QUIT. I AM NOT SURE WHAT COULD BE YOUR PROBLEM BUT IF YOU ABSOLUTLY NEED TO GO THAT FAST (WHICH IN A Z YOU DO) I WOULD TAKE IT TO A TRANNY SHOP
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