Italian tuneup means thrashing the heck out of the engine for a while to get it as hot as it's ever going to get, and breathing lots of air too.

Now the fact that the engine is apparently up to normal working temperature when the fault occurs, rather than merely "not cold" tends to militate against the otherwise convincing theory above of a three-position enrichment process skipping its middle setting. How far are the shops?

I may be wrong but surely a 1978 Fairmont will have carburettors rather than electronically controlled fuel injection, so the diagnosis process may be rather different from the OP's Honda.