Diesel Don,

My son went through this problem for months with his turbo 300 and eventuall found that the surge tank had a small crack around the neck of the tank and the radiator system never pressurized, therefore the boliing point of the system went up dramatically. Since he replaced the tank with a new and highly ribbed one, his car runs as cool as both my 300D and my daughter's 240D.

Check for cracks, now. If in doubt try removing the tank cap and putting your mouth over the neck opening and blowing into the tank, if you can't maintainthe two PSI that your mouth can produce, then the tank certainly can't maintain the 13PSI that it needs to maintain.

Ben