If you are having an overheat condition at low speed, wheeling, idle then I would consider replacing the mechanical fan clutch. When it becomes weak, it may "feel" ok, but possibly not draw enough air over the radiator.
Another thing to consider is that your radiator may be plugged with bugs and mud between your a/c condenser and the radiator itself. Hose it off it that's the case - and inspect the radiator fins while you are at it.
It is possible that you have a defective rad cap. OEM is 16psi and Stant is a good aftermarket cap. Likewise your tstat could be defective, and for 4 bucks, putting a 195' tstat in could also resolve the problem.
I'd focus on the mech fan first.