I have been having a similar problem after replacing all hoses and radiator, water pump and thermosstat, I have NO loss of fluid however. Radiator was leaking and after many months of delay finally replaced everything. Now I can keep my 95 SLS(173Kmi) at 196F with easy driving but if I am pulling hard or going up hills it will climb to 223F, it cools back down if you go down hill etc. I can go on the highway at 70mph and maintain 196 - 199F no problem. Any thoughts?