Here's a puzzle. Removed alternator tests fine in shop (twice removed and checked). Installed, with battery reading 12.4 to 12.5, starting and running at idle indicates voltage is about 11.8. It does drop to about 8.5 when preheating (glow plugs) prior to starting. Quickly turning ignition off and back on will show momentary voltage reading of 13.7. Revving the engine will sometimes (not always - it may take 5 or 6 attempts)cause voltage to increase from 11.8 or so to 13+ and hold. At that point idling the car the charging will continue and slowly build to 14+ while generally in the area of 13.7 I've checked connections everywhere, belt as well, and am totally stumped. Can anyone offer any advise. Thank you.