hi there just to throw my 2 peneth in, i had the exact same problem on a 200 tdi 110 the guy had done everything you have done to try and cure it to no avail, when i looked at it i used my digital infared temp sensor and found when the gauge read hot the engine was about normal changed the gauge and all good,normally with this amount of constant overheating something will give up if nothing has failed and you dont ever lose water i'd look down that route good luck