My 2006 Rexton 270 has a intermittent problem the heat gauge drops and the yellow light comes on, no power loss. after a few km all is ok.

Maybe air trapped in the cooling system. Are you loosing coolant? Look in the radiator, is it full or can you see the core? If you can see the fins, find the leak. You can look for wet spots after it's been ran wherever one piece of the cooling sys connects to the next. Fix leak, now when you refill your Coolant, pour it in slowly to allow air to flow out as it fills up. When it's full, let the engine run with the cap off, and you'll see bubbles pop up. Rev engine to 1200 rpm. Will also help. Just don't get crazy with reving up to higher rpms at idle.

NOT THERMOSTAT!!! its too fast to be thermostat, I think is false contact electric... checked wires from the temp sensor to the harness and nothing out of the ordinary... did it a few times (temp droped to 0gauge, SES light on... then temp rised to normal SES OFF, gauge moved at the speed of switched on.) nothing changed. haven't done that since.

Sounds like the thermostat on the radiator has something flowing thru and jamming it - Do a water flush with chemicals to clear out the junk. and buy new thermostat - about £6-8 max refill with water and antifreeze to improve cooling efficiency - 2 days before flushing put cleaner fluid in the radiator not the overflow bottle as it may not get sucked up. Then do the flush after a day or two as all the gunk will be in suspension, and should fix it.