1987 Ford Bronco

I have an 87 Bronco II that started doing exactly the same as you have described, running fine until warmed up, computer would never go into closed loop. The first mechhanic missed all the ground problems & told me the PCM was bad, wrong ! Mine was resolved with the following findings by a 2nd mechanic, negative battery cable attached/grounded to exhaust manifold, moved to front of block, orange ground wire to PCM was also attached to same location, moved computer ground to proper location on rear of passenger side cylinder head, ground strap from rear driver side cylinder head was looped back to engine instead of being attached to wiper motor.