Driving from SF to Salt Lake, heater stopped blowing hot and temp gauge went to H so I pulled over. Temp gauge dropped back to normal in just a few seconds. Drove on and was fine for a few hours. Happened again, pulled over and heard gurgling sound from heater core, heater started blowing hot again and temp gauge dropped back suddenly to normal. Drove an hour more, happened again same as the second time. No leaks can be seen, no steam, and dip stick has clean oil…???