MY 07 Phazer over heats,,,,,help meeeeee

[SaskPhazerMan]

Hello all. I went out on the weekend and my sled overheated like 10 times. Its was minus 25 Celsius and there was no problems for the first 15 miles or so going across a lake. Then the light came on so I stopped and let it cool down. It overheated approx 5 times on the way back. I was just out in the garage with it trying to figure it out and I thought I might have had like an air block in it so I filled it back up, ran it and got it up to temp, filled it up with new anti freeze and them after that I took the rubber cap off to let out any air. Once just anti freeze was shooting out I put the cap back on. It ran perfect for like 5 minutes then overheated. I could see some antifreeze in the reservoir moving so I assumed the thermostat opened up and then all at once I could see the antifreeze filling the reservoir right up to the top and then the overheating light came on and it started overflowing out the hose. What do you think the problem could be? Thermostat? Air block? Why just suddenly like that after like 5 minutes? Shouldn't a 4 stroke with a rad be able to just run without overheating if it is just sitting there and not moving? Some one give me some ideas on what to do so I can go sledding this weekend please. We only have like a month and a bit before it starts to really warm up and sledding is over. Help?