May be blowing cold air for a number of reasons, sounds like the main reason is it gets low on coolant. So where does it go? What does the exhaust look and smell like? A-lot of white smoke with a sweet smell? Also check the oil to make sure it is not going in there. Dip-stick will show over full.
If the coolant is not going into the oil or out on the ground, there is only one place for it to go and that is out of the tailpipe. Bad head gasket or cracked cylinder head would be the most likely cause. Pulling the spark plugs out may help you find the cause. If coolant is going through a combustion chamber the spark plug in that cylinder many times will be noticeably cleaner than the others. Also make sure you don's have a small coolant leak that is dripping on the engine or exhaust and evaporating before getting to the ground. Checking the coolant for combustion gasses may be prudent and kits are available at parts stores.
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