Okay, a little background first

The engine in this car has had this problem since it was brand new. The car is actually a 1990 240DL automatic sedan, but the drivetrain is out of our old 1989 240DL auto B230F which gave its life in an accident to save my Wife's life and that of our Boxer. The front end is three feet shorter than it was before, but the engine was still running and all four doors still opened and closed.

In any case, the engine has been completely rebuilt (sucked in dirt during accident) and all cooling system components have been replaced with NEW parts. This includes:

Thermostat
Heater Core
Heater Valve
Radiator
Overflow tank and cap
All hoses
water pump
fan clutch
fan blades
aux fan (added)... yes, pushing air the right way
Block and head have been flushed and cleaned to perfection, yes, even the water pipe running along the passenger side of the engine!

Now, for the details of the problem. If the weather is below 85F, there is NO PROBLEM AT ALL. Up around 90F, with the AC running (yes, condenser is new too) it begins to overheat after about 3 minutes of idling. The temp gauge, over this 3 minute period, goes from dead center right to the white line below the orange overheat area.

Once the overheat condition occurs, I can rev the engine, lets say to 2500RPM and get a slight reduction in temperature. If, however, I crack open the heater valve for as little as two seconds and then close it again, the engine cools right back down for another two to three minutes... cycle then repeats!

Keep in mind, this engine has had this problem from day one when it was new off the showroom floor.

In my humble and semi-educated opinion, it seems like the coolant is boiling in the head and that "cracking" the heater valve open provides a flow path for the gas to escape which results in the measured temperature dropping.

Out of these observations the first thoughts that come to my mind (or whats left of it after restoring this car), is that there is insufficient flow from the waterpump to "overcome" a "vapor-lock" type situation in the cooling passages of the head. This leads to thoughts like:

Is there a higher pressure (narrower clearance) water pump available?
Is the idle simply too low at 750RPM.
Should I try to "rig" a different sized water pump pulley?

Thoughts, comments, and or curses will be appreciated!

Thanks,
John