random flat battery

A diode failing doesn't seem to make too much difference to the charge - probably only at high output. But you've ruled that out if you've done a quiescent load check - a short circuit one shows up as several amps. And diodes tend to either work or fail - rather than go intermittent.

I was trying to say: I have seen faulty alternators that drain the battery yet still charge ! most common problems are incorrectly fitted radios or accessories and the old favourite the interior light, boot light, glove box light and similar, try taking the bulbs out of all of them. Weirdest one I ever fixed was an escort mk3 where the starter motor solenoid drained the battery , never seen it before or since. The other thing that happens on some cars is where people leave the keys in (car in garage) which sometimes powers things simply because the key is in the ign. If the key is left in the accessory position on one of my cars the battery is quickly flattened by the ABS pump running every once in a while.