Noticing on my 2000 Bravada that when the RPMs climb above 4000, the voltmeter drops pretty sharply - down to about 11 volts or so. When the engine speed decreases, the volts come right back up again. Also, if the AC is running, it stops blowing cold as well.

The only common thread I'm seeing is the serpentine belt. Could this be indicative of a failing belt? It looks fine by visual inspection and there is no squealing or burnt smell that would indicate slippage. Thanks!