coolant loss but no internal leakage

I have a 97 crown vic. 4.6l
3 mos ago, the intake manifold was replaced--old one split
about month and 1/2 ago, I notice that the coolant expansion tank was
dry. I have been adding coolant ever since.
I pressure checked the system. the needle holds steady.
No external leakage of coolant noted. water pump was replaced about 5 mos. ago.
notice from time to time, that the heater will blow cold. the temp gauge will climb, then fall back down--heat then felt coming from vents. still happens even after replacing thermostat.
I'm stumped. If the head gasket were failing, or if I had a crack in the block, then I would have seen a loss of pressure when I pressure checked the cooling system.
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Is your heater core ok ? How about the hose under the manifold that connects to the water pump ?
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* Steve Stone:

the car. Hose under the manifold that connects to teh water pump? When I replaced the pump, I do not recall any hoses connecting the manifold to the water pump.
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