I don't know what the fuel pressure is on the 3.5L but the 4.3L had over 40 psi making vapor lock imposible.
I'd say there's a problem with the fuel pump, wiring or there's dirt in the tank that get's disturbed when you fill up then settles.
Talk to your service repink.
Sh)t i didn't even think about that one. Dirty fuel tank/filter is a very possible.
