My 1991 Nissan 240SX (only 85K miles!) has an intermittent overheating problem that began last November. Needle usually right in middle, but occasionally would quickly rise to danger zone. In Nov., I got new radiator and pump, as well as all new spark plugs. Was OK for about 3 months, then started overheating occasionally. In shop a day and a half while they tried to find the cause. Gave me a new thermostat. Then test showed there was a leak in head gasket. Instead of expensive repairs, I put 8-oz bottle of K-Seal in cooling system. Reviews all over the web gave me confidence it would seal all leaks. But within 5 minutes, it overheated. Tried again later, same result.
You already know the next step.
Sincere best.
TSMB: /concur.