There sounds like a problem with the 5 volt reference circuit. It may be a bad ECM; but, this video is very interesting, the guy is saying a problem with the 5 volt reference circuit could be a computer power or ground issue, or caused by a short in a sensor. When I look at some of the diagrams, it looks like sensors to also check are the MAP sensor, ECT sensor (which you replaced), TP sensor, and IAT sensor. In the service manual, the harness wiring is mentioned as something to check too.

Warning: This is a long video; but, very technically interesting.


If you had an ECM to substitute, it would make thing easier; but, getting one from someone to test without paying first is not easy to do.