Oil guage dancing at idle.

I would say the guage it not good anymore. I would try to find one at a salvage or wrecking yard and replace it.

If the gauge is not good any more, why would it be ONLY problematic at idle and hot? It seems to me if the gauge was bad, it would be problematic both hot and cold at idle OR problematic all of the time.

If the gauge is only bad at hot and idle, then at these miles it does not seem sensible for me to worry about changing it until it quits working altogether.

You really need to put a Manuel guage on it before replacing any parts

Is there anywhere on the engine, other than at the sending unit, to attach a manual gauge? The sending unit is a royal pain to access, so would rather attach manual elsewhere, if possible.

No you have to go through the sending unit. And if you are going to go through that trouble just install an after market gauge while your there

I would definetly use a manual guage and check to see if it really is flucuating. If so then you have an internal engine problem. You can add lucas oil stabilizer to buy you some time.

There is no way that there is an internal engine problem for two reasons.

First, the fluctuations are instantaneous, like flipping a light switch on and off. Oil pressures do not fluctuate that fast.

Second, I have driven it like this for well over a thousand miles, both at highway speeds and in stop-and-go traffic around town without any other problems whatsoever.

My thoughts are that this is caused by a short somewhere, but the fact that it is a selective occurrence seems to rule that out.

As I have already said I believe it to be the guage (cluster). It don't have to be a short in the wire it can be the cluster I had to replace mine in my F-150 oil pressure guage just flickered then just quit. If you would unplug the sending unit and with the key on ground the wire if the guage pegs then wiring is good.