I though you said it was a broken *main* ground strap? Have you sanded down and reconnected all the main grounds? One at the battery, one at the block, and depending on the car, one more on the passenger side strut tower.
The coolant temp sensor moving should be a clue: there are two sensors on the M20, one for the motor and one for the ECU. Maybe on the M50 it uses the same sensor? If the ECU thought the coolant temperature was significantly higher or lower, it would throw the fuel mixture into disarray. At fully cold, my tune requires 160% of the fuel than at fully warm, for instance.
Imagine the motor suddenly thinks the motor is fully cold (or worse, -40 or whatever the sensor's floor is) or overheated (160 is the sensor ceiling on the m20) and pulls / dumps in fuel. It would run like shit.