Low coolant at idle

[br44]

I have a funny little conundrum that I cannot figure out. The title pretty much sums it up. Every time I start the truck nowadays I have a Low coolant light pop on. I let the truck warm up and it stays on. Once I get goin, The light goes out. When I stop at a sign or light, It turns back on. Thoughts? I am thinking a plugged up radiator or bad thermostat. The light is only on at idle, and the reserve and rad are full.

EDIT
oh yeah... and i discovered this right after i did a quick fix on my heater core leak problem. ya know... the 3/4 to 5/8 connector thing
btw the fix works like a charm (no more green puddle in my car)

 

[Oldsouthernboy]

On the left side of the radiator as you stand on look at it, about mid way down is the sensor, It will have a black cap with wire from it, you may have a dirty or loose connection to your sensor, clean it and make sure you have a good connection, or replace the sensor costs about 8.00 for the new sensor. This wiring comes off the right head light assy, under the front battery, some times I have found the sensor end cut off but the end is grounding out on the frame as you drive.