My coolant keeps Disappearing

I have a manual 1998 Pontiac Sunfire. It's a 2 door and has like 129,000 miles on it. I was out of town for a week and half and hadn't driven it for that time. When I got back I noticed the temp gauge kept surging up and down and went into the red a few times. Then one day I pulled into the driveway at work and there was this gurgling sound on the right by the passenger side. I later checked and there was no coolant left. My roommate and I thought it was the thermostat so we replaced it. My car was fine for like 2 and half weeks and the temp gauge stopped surging. Now it's completely schizoid or somethiing and is moving up and down constantly. I've put like 3 jugs of premixed antifreez in there the last 3 days. What's wrong w/ my car?