Other than the relay itself. I do not know how it senses if the engine is hot or not. If the relay senses by whatever method the 1985 uses that the engine is already hot the relay will drop out quickly. Find out the system it uses and check it out it should be easy to do.

A salvage yard may turn out as a good source of a replacement relay as well if required. It does sound other than the external sensor circuit possibility it is the relay. Some of our guys that advertise used parts on the parts section might have a spare one to sell as well.