Thermostat opens at 180 on the stove.
When does the light and alarm go off? After 10 or 20 mins of operating or right away? I wonder if its a problem with the sensor/sender for the alarm or a bad wire that has had the insulation worn through and is touching ground, or it could be a bad ground in the wire harness.
Anyway you can fit another guage or pick up a hand held temp gun to verify how warm the engine is when the alarm goes off?
The coolant is normal looking I guess? if you blow a head gasket it gets all oily and grey but you can have almost instant over heat problems.
Does the rad seem to get hot? Is there a hot breeze from the fan? The fan shroud is still there and the fan belt is tight insuring good air flow though the rad.
Rads get plugged inside and don't transfer heat, if you spray the rad with a mist of water from a garden hose the evaporation of the water will cool it down, if this help the heat may be getting to the rad but not out into the air from the fan for some reason.
Does it only overheat when working it and cool down when you leave it it idle? or once its hot it just keeps getting hotter? (won't cool off even if you remove the load from the engine?)
Ken