2000 Mercury Sable

A few months ago I lost heat in the car so I checked the coolant and saw that it was low. I added some more and the heat went back to normal in the car. After a week, I lost heat again and I saw that it need more coolant. I checked for a hose leak and found nothing. Also the engine had been operating at normal temperature. A few days later, the car stalled and although I was able to make it home, the car ran poorly and hotter than normal. I understand that this might be a blown head gasket, but it doesn't seem to have very many symptoms (no smell, no coolant or water in the oil, no smoking). I'm assuming the car has lost manifold vacuum because I lost power in the brakes. I was told it might be the intake manifold gasket, but if this is the case, where is all the coolant going? Thanks for any help you can give.