Originally Posted by Argess

A leaking cyl head gasket can allow cylinder pressure (i.e. exhaust) into the cooling system. This can pressurize the system forcing coolant out the rad cap until there isn't enough left to adequately cool the engine. Sometimes these leaks only show up when en engine is hot, but any rad shop should have a pressure tester they hook up in place of your rad cap. The pressurize your cooling system and make sure the pressure holds and doesn't drop off.

When I fill a dry engine with coolant, I try to remove something high up to help air bleed out. In my case, it's the temp sender unit located on the intake manifold. This shouldn't affect you as you've driven the engine long enough any trapped air bubbles in the cyl heads should have cleared on their own by now. So that doesn't help you at all.

Next time the engine is fully warmed up, besides looking into the rad, check the bottom rad hose. Be careful of the fan, but rev the engine a little while looking at the hose. See if it starts to suck in (collapse) as the rpms come up. You don't have to rev it high...maybe 1500 to 2000 rpm for a second or two. Maybe keep the rad cap on while doing this lower hose check.

Originally Posted by Argess

That's a good idea. I made an airline connection for doing leak-down tests by welding a fitting to a sparklug shell (porcelain knock out). I coudl use that next time I think I have a head gasket leak. Maybe the OP could make one too (if he has an air-compressor and a welder). It would certainly show up leaks, although there's only a pinhole in the thermostat if its closed, so maybe after the engine is warmed up a bit.

This thread is getting fairly long, but I think someone did mention "fanbelt" and I'm sure the OP has checked it. However I have had fan belts slip even though the tension was correct. They get glazed, or the pulleys get glazed or something.