383 running like s#%t.

I too vote fuel.
It's not uncommon for the IR gun to give erroneous readings;selecting the target is often critical.But your temps do seem to have a pretty big spread.
I'd like to think it's something simple like random water slugs moving on thru. But there's just so many things it could be. If it seems to always be the same cylinder, that would make it way easier to diagnose.
But you know what, I have seen that kindof thing happen when the engine is given a bit too much timing as well.