Voltage drain issue, perhaps a backfeed?
 

1994 Accord, EX, 2.2 VTEC. Replaced motor after it had been sitting 5 years. I have an electrical issue. New battery, new alternator, car drives fine, but when you drive over 40 or 50 miles, the guages start fluctuating wildly, indicators go dim, no radio, ABS compressor(?) goes on and off repeatedly. Turn the car off. It wont crank. Battery is dead. Jump it off, drive another few miles no problem. Battery and alternator have both tested good. Advanced Auto test says there is a "diode short" causing a 119mv ripple. On this vehicle I can find one diode in the wiring schematics, the A/C diode. Of course there is also one in the alternator, but it tested good at 2 separate places. Could this A/C diode be the cause of the electrical drainage issues? I am above average when it comes to reading prints and drawings, but this one has me stumped. I dont want to break my back to get at this diode(Im 6'2" 260) if its not even the cause. Any help would be greatly appreciated.

I have had somewhat of a similar problem and had batteries and alternators tested good in three different places and it turned out to be a starter drawing down the battery. It could still be a bad cell in the battery,try swapping the battery with another car you own or a friend could lend you one,If not that,I would replace the diode, that seems like the most obvious.