A (#17) & B (#18) are the 2 fan switches. From your description it sounds like the circuit controlling your fans is OK.
#15 is the sender for the gauge.
#19 is the coolant temperature sensor for the ECU.

Look for an intermittent short to ground in the wire for sender #15. That would give a bogus high temperature signal to the temperature gauge. Or maybe sender #15 is just going bad.

Check the coolant in your radiator. It's possible to have air pockets in the radiator & hoses, while the reservoir bottle is still full. In this case the sensors will see strange things.