My 1997 F150 is overheating.

This summer I changed my coolant (I hadn't done it in quite a while) The
coolant appeared to be quite rusty and it took several flushes to get it to
look normal.

About a week ago my truck overheated the first time. I thought that the
thermostat was sticking, so I took it to a Rad shop had the system flushed
again, thermostat replaced and heater core flushed.

On the way home from the shop it overheated again, They took another look
at it and found the pressure had created a leak in an extremely rusted
block heater, it was replaced.

It overheated again, they then did a test to see if the head gasket was
leaking the test didn't confirm anything. They changed the water pump
thinking that corrosion may have damaged the impeller.

Well the problem is still happening, it appears that when the motor cools
down it is getting an air lock somehow in the system, when warmed up the
temp gauge fluctuates in the normal range but will not produce good heat in
the cab, after about 10 minutes the temp gauge spikes and the overheat
light comes on. When I check the engine the upper rad hose is not
pressurized, if you open the overflow bottle it releases the airlock and
then the vehicle operates at normal temperature with great heat.

There isn't any coolant in the oil and it doesn't appear to show white
smoke at start up or while running. With it overheating it always seems to
loose a little at the overflow bottle so it is hard monitor coolant level
from test drive to test drive.

Any help would be appreciated.
What could be causing this air lock?
Has anyone had head gasket problems I would hate to spend the money on this
if I do not have to (estimated @16 hrs job)
I have 260,000km (162,000 mi) This truck hasn't cost me a dime except for
maintenance till now.