I will agree but restate
Idle/low speed heating is typically fan related. Be aware that Electric fans are ususally overated cfm wise and the performance of the fan is usually directly porportional to the price of it.
If it's getting warm above 30 to 40 MPH you have other issues as the fan should be out of the picture unless it is postioned as such that it is blocking the air flow.
I usually start with a digital thermocouple guage and probe inserted into the coolant as close to the thermostat as possible. This allows me to see exactly what is going on. I do not rely on infared meters as they can be inaccurate.
I have seen brand new gauges etc be off by as much as 50 degrees for various reasons
As previously mentioned if it's getting warm at low speed/idle and OK going down the road look at the fan
If it's getting warm going down the road
Make sure the lower hose is not collapsing,
Make sure your pulley ratios are correct,
Make sure the belt isn't slipping

Check for restrictions to the air flow

Check for air flow escaping by going around rather than through the radiator

Long shot but I have seen loose impellers on the water pump

make sure there are no vacuum leaks and that the motor is not running lean,
Timing being to far advanced at low speed or at higher speeds

block check the motor to make sure there is no internal issues

If all of the above checks out then it may be time to start looking at the radiator itself