I also have a 1993 Ranger. I am dealing with an issue similar to this. We had the freeze plugs blow out on us during the summer (I know right?!). Our cousin replaced the freeze plugs and put a new radiator in. We got it back and the temp gauge is going up and down from the N in normal to the L and back again. It never goes past the L.

I replaced the thermostat, with the assumption it was going bad and was opening late and closing early. It did not work

I replaced the fan blades. In-between every blade the plastic was cracked. My thought was if the blades made the clutch off center it might not engage correctly and cause it to not get the air needed. It did not work.

I replaced the water pump. At this point I could hear water trickling through heater core area and assumed the pump was going bad and was not pushing the amount of water needed. It did not work.

Last night I was driving home and the temp gauge spiked to "/H" and did not return down with in 3-4 seconds. I cut the engine off and when I did, I heard GULPING sounds from the heater core.

I replaced the heater core this morning. I do not hear the trickling or the gulping anymore but it still goes up to L but not "/H". It did not work.

I only have the fan clutch, overflow hose, and overflow tank left to replace. **aside from the sensors themselves**

Notes on truck:
It never does this at idle.
There is no specific speed that it does it more or less at.
Often times after this happened I open the hood to find the connection from the overflow hose to the overflow tank has expelled coolant. *Not a lot but some - tank is still 1/2 full*
When I open the radiator cap after it has cooled (warm not cold) it releases pressure and I can hear it blowing air/air bubbles into the over flow tank.
After it does this the radiator cap is cool to the touch but the radiator is hot.
When I open the radiator after this has happened and the engine is cool I need to add 1/4 to 1/2 gallon of coolant.


Any thoughts?