I have a vapor lock in the cooling system that I can't fix. I changed the thermistat because I was getting very little heat, that worked for a while, then April of 2010 I couldn't get any heat from the front or rear heater, it was like that all summer. The cooling system worked fine just no heat. In August I pulled the dash apart to check the controls and ducts and they worked fine. I wormed the engine to operating temperature and the heater core was cold, so I back flushed the system, and still no heat. I opened the bleeders to get the air out, I would drive a mile out and back and bleed the system again, and I did this 10 or 12 times and finally the heaters worked again. That lasted about a month and the temperature gauge fluctuated and the heaters quit working again. I bled the system and the heaters work for about a month and I had to bleed it again, now that the weather turned cold the heaters work intermitently and I have to bleed the system almost daily. What can I do to fix the problem.