My father in law has a 2002 GMC Sierra Denali and it wants to stall at times, and sometimes does stall.  It's an automatic. It has almost 350,000 km on it. There are no codes and no check engine light on. He has a good, well trained and experienced mechanic who told him the last time I witnessed that "those 6.0 L. Engines are known for it" but had no further advice..

As far as i know, he had the battery and alternator replaced with new parts, the injectors were cleaned, and he recently replaced the exhaust manifolds.  I know he's used gas additive injector cleaners before.

He's away so I took it for a drive and i noticed a few things:  1) i thought he was an aggressive driver, but this thing takes off as soon as you can get the accelerator pedal to move just a bit.. I'm wondering if it's been tuned.. 2) i turned around and came back and when I stopped to back into the driveway to it was idling rough around 500-600 RPM (seems low) and the battery gauge was jerking from 14+ down to 11-12Vdc at the same time as the idle weakened.  It actually stalled halfway through backing up into the driveway during the initial turn.

suggestions I've seen online are vacuum leaks, battery and alternator issues, and injectors and throttle bodies.. any ideas?