its got to be the calipers. the rubber seals in the calipers are old and get hot and lock up. i'd say its time for a new master cylinder and new calipers. my brothers 85 celica does the same thing and he has steel braded lines on it already, so we started replacing calipers. but the therory of the car rejecting the 1j is a good one. heheheheheehhe or maybe the car is afraid of going really fast, your lucky, my mk3 loved going fast, but didn't like to stop.