Amp draw causing 4L60e problems...

My '98 GMC has VERY RARE occasional TC slippage - I know that ONLY because it will trip po1870, and equally as rarely it will max the tranny fluid pressure and cause the hard shifting.

It happens only in stop and go traffic, A/C on, fan max'ed. And extremely rarely at that - a couple of times a year.

I'm convinced it's an electrical problem, not tranny/mechanical.

These are the only problems I ever have (190k miles, maintenance is TIGHT, everything).

Another electrical phenomenon I can create which I think might be related - with a half a tank or less gas, if I coast to a stop in neutral, the gas gauge will drop to '0'. Do the same thing in gear, it holds steady. Nothing is different except current production at the alternator - right?

Gotta' be tied to the tranny problem...

Ideas?