Now I'm back to thinking air in the system.
I drove it twice today. One for about 25 miles each way, and once for about 10 miles each way.
On the first trip, no problem on the outbound, but the needle pegged and light came on on the return trip - though it didn;t seem to be running hot.
On the second trip, just the opposite - needle pegged and light on on the way there, but no problem on the way back.