Engine running cold (per the gauge), rough at low RPM

[454cid]

One of the smaller shops was able to determine that the thermostat was missing its outer seal/gasket

Lol, so I think that means the thermostat was just rattling around in there, assuming your truck uses the rubber gasket.

The gauge still looks like its running colder than normal (well under the 210 center point), but it's been ungodly freezing in the Midwest recently. Next time I find a good price on an IR temp gauge (probably Harbor Freight) I'll pick one up and test it.

Some of them just read low. I want to get an IR gun too. I've borrowed them before... sure are nice to have an make neat toys.