Seem like the same problem as I had.

Found out, after much drama that it was the Head temp COOLANT sensor (not temp gauge), located on the passenger side of Cylinder head, the rear most item, below the rocker cover, with 2 push on connectors.

To test, pull, carefully, the 2 leads off the sender leads (Brass and usually green with 2 ceramic posts), and put an ohm meter into each post. if NO resistance, somewhere from 400 - 1.2K ohms, usually, then this is your problem.

Not easy to get to, needed to 'chisel a corner on the brass to get it in. To get it out use an open ended spanner, 21mm on its head to remove, a seal ring will be attached.

Also check your distributor cap and rotor button, for Carbon build up, and intermittent firing.!