Antifreeze disappearing

I have a 1993 Crown Vicki and for some reason this week the antifreeze has started disappearing. I've filled up the reservoir several times and I know the radiator is full. I will run the car for a short distance and the antifreeze is gone from the reservoir after sitting for about a half hour and does not reappear. If I put more in and then drive it again and park it then it overflows. There's no puddle and it's not in the oil so I'm stumped as to what's going on. Put in a new thermostat and that helped with the temp readings. Any suggestions?? Thanks in advance.

Loan a tool program at most auto parts stores. Pressure tester.