Check your ECM and do a "tap test". If your original ECM has GM service # 1227727, the ECM may be the problem. With the car running, gently tap the ECM. If the engine stumbles or dies as you tap the ECM, you found your problem. That particular ECM mentioned above has a MUCH higher failure rate than most GM ECM's, partly due to it being mounted under the hood, and partly due to it's design. Normally, symptoms include random loss of injector pulse, spark and crazy engine temp (-40 deg), etc... Just for reference, when doing the jumper wire test, a code 12 indicates no distributor/spark reference. Since you're doing a key on/engine off test, this has been universally interpreted as a start of the code sequence, but technically, it's telling you there is no spark signal which is normal since the engine isn't running. It doesn't show up on a scanner test, because the scanner knows the engine is off and automatically omits this code to eliminate confusion.