1999 Pontiac Montana

here in the last month I have had to put antifeeze in my van 2 times. Every once in a while see a tiny spot on the ground. Today when I drove it, the thermostat guage started going up and down. It went all the way up to the red line, and then back down, then it went up a couple of more times, not all the way up to the red line
but close. I drove it less than a mile. Not sure what to do.


Sorry forget a bit of information 4 years ago we replaced the top half of the motor. the milage on the van is 150,000.