In the powerfc, go to ETC > Sensor check, do that first while the problem is evident (when cold).
Look for any values highlighted with a black box around them. If any are highlighted, the parameters are incorrect, which means faulty setting, sensor or wiring. Your exhaust temp light should be on if there is a fault, although considering the engine is transplanted then it may not be connected.
If there is nothing, it could be your air reg. It could not.