1991 Toyota 4Runner TEMP GAUGE

Engine Cooling problem
1991 Toyota 4Runner 6 cyl Four Wheel Drive Automatic 120k miles

RECENTLY MY TEMP GAUGE WENT TO MAX TEMP RANGE. THEN WENT DOWN TO ABOUT MID RANGE. AGAIN THE NEEDLE WENT TO MAX RANGE.

FLUID LEVEL. NORMAL TEMP OF THE ENGINE (WAY IT RAN, SMELLED AND FELT, THE ENGINE WAS NOT OVERHEATING) IS THIS A SENSOR PROBLEM OR?

I MIGHT ADD. IT DOESN'T ALWAYS HAPPEN.

THANK YOU IN ADVANCE