Wacky temp light on '84 4KQ

[Carl & Dawn]

Well,

I flushed the engine and radiator and now the temp needle barely budges
past the 1/2 mark.  When standing still, the needle goes between 1/2 and
the first upper mark and the fan kicks in and out as expected.  However,
the temp light will blink until I get rolling again like it is
overheating but according to the gauge and inspecting the overflow tank
for boiling it is not.  When I unplug the sender for the gauge on the
outlet housing ( I think - it is where the upper rad hose connects)  the
blinking stops and I lose my gauge.

Any suggestions????

TIA
Carl