Sorta new here so ......Hoosier1 or Bob,
Seems as though you might be dead on about voltages causing puter not to work to specs. Mechanic friend speculated that the voltage/amps dropped after a 30 min. drive due a faulty alternator and low battery. He says that the power required by some systems (radio, tape deck, abs, horn and lights) cause them to trip off-line before the puter and finally even it goes south but, prior to total shutdown the onboard puter is suffering from too little juice and it just gets flaky. Since the trannie is controlled by it (puter), it flakes also. Gravel pit!!
Had wondered and even checked the codes to no avail. Seems that the puter that providing the codes is so low on juice it didn't note the irregularties. While I didn't get any codes I haven't had the problem since I replaced the alternator.
I thank both you and my mechanic friend for "connecting the dots".
Variable