Does yours have an auxiliary fan? Single row or double row radiator? Air conditioning?
Usually higher temps. at or near idle indicate a marginal fan clutch.
Most of the air is force fed at normal road or highway speeds. More fan often doesn't help much, if at all, at highway speeds.
Check and see if you can squeeze the inlet radiator hose partially closed at 2000 RPM on a warm motor. If you have flow resistance in the system after the thermostat, the inlet hose will get really hard from the pressure if the radiator is partially plugged and may even visibly swell as the RPM's go up.
The thermal switch that turns on the Aux fan is in the lower left of the radiator. They don't often go bad, but they can get lazy. Check the plug on your aux fan. Like I said though, the fans don't actually do much at highway speeds. On my 88 the fan clutch took a dump, I removed it and drove without it for a couple of weeks, no problem as long as I kept driving, slow speed and idle is where it is really needed.
Flushing an older radiator can be iffy, the copper rots from the outside, acid from decomposing pollen and other biological junk. And the copper leeches on the inside as the coolant changes PH from age. In other words the copper gets thin and a flush may cause leaks or seeps on an older radiator.
The coolant system has to be leak free, anywhere that lets the coolant out can suck air in during a cool down cycle. Air inside the system can inhibit coolant flow. And any pressure loss lowers the boiling point of the coolant, if the coolant boils it can also inhibit coolant flow. Tiny coolant seeps can cause large problems.