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.