Temp gauge bounces from hot to cold

A month ago my car temp gauge was registering hot. It would bounce up to almost Red and back down to normal in the blink of an eye. I noticed it happened when I stopped and went to normal when the car was moving. I checked fans and they were working fine. I then had the radiator flushed and changed the thermostat and that seemed to work for about three weeks. Now I noticed a few days ago that the gauge would do the same thing, bounce up to almost red and then down to normal in a matter of 5 seconds. Now it happens even when the vehicle is moving. I don't hear any noises such as a water pump bearing, and I checked fluid levels. I commute 70 miles a day an worry its going to overheat. What should I do now. Someone suggested it could be a head issue, but I don't see how, what are some possible issues?