Tried to search for this but its kinda weird. Figured its best to let ask before i get lost in diagnostic hell.

2 days ago got stuck in a massive downpour driving home from work. Yesterday car started fine so went for a drive, after about 15 min the temp crept from 1/4 up where it normally sits to halfway then wouldn't rise any higher. Once the temp rose to half started getting a consistent misfire, no hesitations or backfires, just felt like a plug wasn't firing every 3-5 seconds. Let the car cool down and didn't notice anything visually wrong, after it cooled down the exact same scenario happened, 15 min in the temp rose to half and not further followed by the misfire.

1986 GTU. New plugs and wires, just cleaned all 4 injectors, Koyo radiator, ports wired open. New oil, coolant flushed a couple weeks ago.

 

Check your coolant level to discard you are in the beginning of blowing a water seal.
Check the TPS, O2 Sensor and Coolant sensor for any abnormalities
Remove and inspect your spark plugs to discard any abnormalities