Curt,
You’ll notice what Chris (V Mad) said, was the head gasket, not the intake. The head gasket can be easily reversed. The gasket should be marked FRONT, which should be placed to the front and up. This accounts for fore and aft as well as right and left. If it’s not, it can cause the kind of overheating you’re talking about. You’d like to think they’d have been sure to check this, but it happens. What Eric said about the spring in the lower hose can be a problem, but not so much at an idle, where you said the temperature really jumped up after a few minutes.
If new intake gaskets were placed with your new intake manifold, which I’m sure was the case, it’s possible that a blockage could be from a "punch out" at a water passage that didn’t release. I’d think this would be less likely and easier to spot than a reversed head gasket(s).
Obviously the only way to know whether it’s a reversed head gasket(s) is to pull the head(s), which is major. I hate to say it, but since the engine stayed so cool before this last teardown, this seems pretty likely. Best of luck, Let us know what you find to be the problem.