Never leave home without an 8mm and a 19mm wrench!!!
I have been experiencing the same issue...I wish someone would offer a permenent solution. My solution was to buy a Mercedes Benz!!! After the 735 got my wife and her client stuck in a undesirable area of a large urban city I bought her an E320 to drive and I still regularly drive the 735. The 735 engine must be bled of an air pocket that forms in the top of the engine on both sides of the thermostat gate. The front bleeder location is perfectly on top of the thermostat housing. Open it with an 8mm wrench, when all of the air has bled close it. (open the coolant filler neck to vent the system as you bleed the air). The second location is the 19mm plug behind the thermostat housing. Bleed here too. If this air pocket forms the thermostat is not submerged in fluid and will not open no matter how hot it gets. If there are ANY leaks in the system it will cause air to be drawn into the system as the engine cools and allow the air pocket to form again. ANY fresh water introduced will contain air which it will release when it gets hot and the pocket will form again. Bleed it again. If it overheats just turn off the engine and bleed the air, when anti-freeze bleeds out of the ports close them up. The thermostat will be again submerged and will then open and you are on the way again. A continual formation of an air pocket will indicate a certain leak, no matter how small, a coolant leak on this engine is a problem. Small leaks are real hard to find because they do not leak when cool and the pressure needed to force the leak is only present when the engine is warm. When the engine is warm the small leaks evaporate as soon as they leak out and then leak the air in while cooling. The solution is the have the system pressure tested while cold. My car sometimes still dispalys this bizzare behavior and I never leave without the 8 & 19 mm wrench, but it never takes more then 10 minutes and a little coolant to be on the road again. Change all of the hoses? Some of the hoses on this car seem to have 4 ends and I expect some have more. Who the hell ever dreamed up this nightmare? Hoses that siamese into other hoses with no hose barb or fittings? I imagine that some of these hoses cost $200.00+. Good Luck