2001 2.3 Temperature/ Coolant Shenanigans

In my effort to get to 150k-200k miles, I figure I'd run into some things like this, and don't mind tinkering a bit. (That said, I'm no mechanical genius- just a guy with some tools.)

I replaced the thermostat a couple years back, and recently the coolant seems to be slowly leaking. Until I track that down, I top it off occasionally:shakehead. Not ideal, sure, but moves.

Last week, while the truck was warming up (40 degrees ambient), the temp guage pegged to max and my fuel mixture went to crap. (Not overheated- it went from 0% to 100% in a second or two, and too fast for the engine to actually heat up.)

I'm guessing check the thermostat first, track the damn leak down second, maybe flush the cooling system (?Hire someone?)... Any advice/ thoughts?

Thanks in advance

The 1998 4.0 litre engine has two temp sensors. One runs the guage, the other the engine management computer. I wonder if yours does and one has failed?