95 LWB put on the road after storage for 3 years all fluids ( including antifreeze ) changed and ran with no problem 1 month. Recent heat wave in northeast and longer commute has caused temp gauge to start to move out of its usual dead center position to the high range after 15 minutes of running. At highway speeds of 70-75 temp drops to normal range. When I slow down for traffic or side roads temp starts to climb. Expansion tank shows no loss, hoses not leaking and neither very soft or hard. Fan has some resistance to moving when engine is hot but moves pretty easily. When temp climbs I put on the heat and blowers and temp drops instantly into normal range. Any direction would be helpful.

 

More than likely, you may need to replace your radiator, I have a 93 RR SWB 3.9 and I just installed my second radiator in seven years. I replaced my coolant every two years and I still had over heating problems.I read an article several years ago about the radiator being a weak point with the Classic RR. Also you may want to check your fan viscous unit to see how much free play it has.
Hope this helps. Ive owned mine for 10 years and I've learned alot and done alot on this vehicle, so please don't hesitate to ask me any other questions.

Regards