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Question: cooling system

I am loosing anti-freeze. I put in a pint to a quart a week in. I see a very LITTLE on the garage floor. I took it to my HONEST & TRUSTED mechanic and he tightened my hoses up and did a pressure test and said all was good. No foam on the dipstick, no film on the window, no wet floormats, no smell.
I will be driving down the street and the water temp goes all the way to H, and no heat, then starts coming down, then the heat comes on, then back up and then back down. I will then go home, fill up the radiator and it goes away.
The only thing I have not done is put a new cap on.
One other odd thing it does is when I start it up and let it warm up for 3-5 minutes it will go from 500 to 1500 to 500 to 1500 to 500 to 1500 revs. Then I hop in and drive and all is fine. Of course this will happen as it turns out when the anti-freeze is low.
Any answers out there???