overheating 300tdt

My 87 300tdt overheats when stopped in traffic. I have replaced thermostat twice, no change. Water pump is 2 yrs old. If I am stopped in traffic in the winter, the heat stops until I start moving, then starts again. if I am stopped in summer, I have to rev engine to 2-3,000 rpms before temp starts to go down. Does anyone have any ideas? I also replaced the clutch fan, no change!