I was going to install my thermostat, but the gasket didn't seem right so I took it back out. Stupid lil me threw the outer thermostat housing into a bucket of water until I realized this was there:

I dried it up, then reinstalled. After a drive, my car started overheating. Any clues?
-Hoses seem fine
-Cap tight
-Overflow bottle boiling
-no noticeable leak from thermostat housing
-had a loss of power in 2nd gear @ 2,000 RPM at a moment

 

ok first things first #12 is the Fan switch it grounds out to turn on the fan.
it should not matter it was under water it's made to get wet.

now if you have a T-stat installed and it over heated on the highway you could have a air pocket. if it happend in traffic then yes #12 could be bad and not turning the fan on.

if you have No T-stat installed and it over heated you have eather a Bad water pump or a blowen Head gasket.

 

Already changed the headgasket a couple months ago, I JUST remembered some coolant came out when I was installing the t-stat so I hope it's only the lack of coolant.

 

top it off and see if it gets hot again and if you didn't deck the block and or used a Fel pro HG then it could be leaking again.