[xeroinfinity]

I would think it to be a fuel pressure issue.
Rent or buy a fuel pressure gauge and test them.
The shradder vavle is on the pass side back fuel rail.
Shoule be caped(looks like an air stem vavle).

Has the fuel filters been changed?
They should about every 30-40k miles.

Both my GA's w/3.4 does this pereodicly, sometimes it even backfires through the air intake.