Check the thermostat to eliminate that as a potential cause.

If you think the cause may be a clog in the rad, disconnect both hoses, drain the fluid from the rad, plug the bottom and pour in distilled water (rather than coolant just so you don't waste any by spillage) and see how much it will take, also see how much comes out of the bottom pipe if you unplug it once you've got a little bit in the rad. If none or very little comes out, keep pouring until you're filling it up. (this would be a check of the hot circuit)

While you've got the hoses disconnected, you could pump fluid through the bottom hose (as that's the way it flows), make sure your heater is fully open to get flow through the core, and check the overall flow (this would be checking your cold circuit). If there's a restriction somewhere, close the heater then try again to eliminate the core.

It may be overkill, but since you've got the hoses off....

I've unclogged radiators using a rubber stop drilled out to fit the tip of a blowing head for a pneumatic hose, place the stop in the top outlet hole, with the filler cap on and the resevoir/overflow hose plugged at the rad's tip and bottom inlet open. Don't go crazy with the pressure, keep it low and slowly increase while giving short bursts of pressure. Just be careful and go slowly. You can also do this with some distilled water in the rad.

All this can be time consuming, but eliminate each cause one at a time. It's worth the effort and time.

Last of all, have you measured the temp of the engine when the guage is inicating hot? I doubt it, but your temp sender could be freaking.