1995 Ford Mustang Car overheating

I have had the thermostat sticking for some time, with it having to go almost to hot before it would come down to normal and the heater would blow hot. Saturday night car was fine, Sunday night the car overheated and wouldn't cool down. I thought the thermostat had stuck again, and tapped where the thermostat would be with no luck. This morning, I again tapped on the thermostat and it went from hot down to normal, I drove it for an hour or so with it running @ normal temp. At the end of the day, I drove it again and had to stop a couple of times tapping the thermostat, finally it went to normal and was fine. Tonight, I replaced the thermostat, but the overheating is stil here. I did not try tapping. What else could cause this.