Heat gauge malfunction even after three repairs

Hello. Back in July my heat gauge was malfunctioning, sometimes working, and sometimes the needle was at the bottom, when it did this, the hold light would flash on my dash. I went to my local mechanic and they put a new thermostat. Every thing was good, then the same symptoms came back, heat gauge on the bottom zero, check engine light on and hold light flashing on the dash. I then took it to my mechanic again, and they put in a new sensor. When the car is idling the heat gauge works, heat is coming. Out hot, gauge on the center line (1/2 way)then as soon as I'm driving, the gauge goes down to the zero and my heat becomes Luke warm. Now the mechanic put a new coil restorer in, and again when idling the heat is hot and gauge is on the middle line, but as I drive down the road, the gauge goes to zero and my heat is lukewarm again. Check engine light back on.

At this point I am very frustrated. My mechanic says it's fine, as soon as I leave the shop, the symptoms are back and I've paid for 3 repairs so far.

What could the problem be? Do I need another thermostat? Or what?

Please help.

Jackie M