300CD runs hot on highway

[mistergood]

I have replaced the thermostat, tried burping the system, all to no great end. When I run the car on the highway, it will still get up to around 100C+ unless I am totally pumping the heat, then still it will be around 95-100. I am supposing that the higher temp is due to the fact that I am running pretty high RPMs at about 65 mph. When I am around town, I have no problem keeping it right about 85C. The thermostat definitely opens when it is supposed to I can see temp drop if I am watching it. Whats it sound like to you guys? Fan clutch(hope not)? Clogged radiator? Clogged rad fins? Any help is much appreciated.