Intake manifold gasket didn't fix my problem, so the heads got pulled
(should have put Velcro on this thing). Put a gauge in the driver side head
and one in the passenger side. Passenger side read nice and normal, while
the driver side was all wacky. -isolated to driver side, at least.
When pulling the head on the driver side, stuff was falling off of it and
there was all sorts of garbage around it. Apparently, what had happened was
some sort of "stop leak" was used to bandage a problem (possibly the leaky
intake gasket) before we bought this three years back. Well, it rared it's
ugly head, as things like this ALWAYS do and I was the recipient. This crap
was in the ports and had made it's way to the combustion chambers. Probably
what was causing the overheating and crud on the spark plugs.
Had the heads rebuilt and a valve job done while they were off, along with
magnafluxing. Painted them the RIGHT color, as well as the valve covers.
Now, it's back together and runs better than it did when we bought it (all
Chevy engines run better when they're orange:-)!
No more fluctuating temp gauge and all's well, since my wife's got her truck
back. She's not a car lady anymore.
Moral of the story: NEVER use a fix-it-fast chemical like this because it
will bite you where it hurts. NEVER "patch" something together and sell it,
like some have mentioned in the very recent past.
About $350 later and hours of pulling my hair out, my lady is, once again,
driving her truck.
Thanks to all that offered advice!