Yes, you're absolutely right. It's the intake not the head gasket. As
for the "near-rough" idle, this is my description of all 2.8, 3.1, 3.4
engines. They seem to have been programmed to idle so lean that the
engine's idle quality is ok at the time of show room delivery, but
5000 miles down the road, things drift and depending on quality of
gas, air temp, atmospheric pressure, phase of the moon, ... etc. they
all tend to idle rough at stop lights with transmission in "D". I've
noticed this only on two brands of cars, GM from after about two or
three months of service and Nissan/Infiniti brands after about 100,000
miles or so. Compare this to the idle quality of a Honda 1.6, 1.8,
2.0, 2.2, from show room floor to the end of life, they usually run
great.

Was there a recall on the coolant flow issue? What year was it
actually fixed in production? I know a friend of mine who had 38,900
miles and his 3.4 in a Pontiac Montana had that problem. Cost was
$3100 and it was not covered