1997 Mercury Cougar coolant leak somewhere

Im sure that im losing coolant because every freakin time I check it again under the hood its low and I know after I top it off I shouldn't have to add coolant after the next few days or less come on lets be real? Now I got some car diagnostics people looking at it to troubleshoot if its actually external or what and to check the computer or whatever they do. But the direct answer to your question is both and what it does is when im driving the engine level temp will go all the way up just about to the HOT level but not all the way and cold air will come in from the blower when I have the heater on and then when the engine fan and everything kicks it will begin to heat up inside and the level will go back down to normal but then after that about 10 minutes the LOW COOLANT light will come on, what u think now?

You know what, I had a guy that worked at F&F tires take a look at it since he's retired and he got it pressure checked for internal leaks and I also had it pressure checked again to see and they both cam back negative so i'm not all that sure but im really thinking on getting the head gasket set and the intake manifold gasket and have him put it on and then try that out because I really cant think of nothing else that this leak is seeping out from. Other than that thanx for the advice man