Have you checked codes by just shorting out the correct pins on the obd1 connector? I don't know the exact symptoms on the 4l80e equipped mechanical diesels, but there is a chance you have a bad engine speed sensor. It caused all sorts of whacky stuff on my 93. Mine gave a code that I could only reference back to cruise control switch, but normally you should get a code 12 with the engine not running, if you get a code 12 with the engine running you probably have a bad ESS.