2001 Ford Ranger
Automatic
4.0 V6
177,000 miles

I get in the truck and start it, about 2 miles down the road the temp gauge is showing as warm, and then it slowly starts to creep up. It usually gets almost all the way to H or the red line, and then it slowly starts to fall back down to the middle (I'm usually slowing down or idling because it makes me nervous). I can drive the truck for another hour all around town and it will never do it again.
If I park for a few hours, and cold start again, it does it all over again.

I don't have any other reason to believe it is actually overheating, so I was hoping someone here could tell me where to start.

 

Welcome to FTE.

Perhaps begin by scanning for trouble code clues & post up All code Numbers as they can help focus a trouble shoot. If no codes, connect your scan tool that can read live PID feeds to the engine computer & have it put up the Engine Cooling Temp (ECT) sensor & see what it's doing.
If you don't have a scan tool that can do that but would like to begin doing your own diagnostics & turning your own wrenches, have a read here about the inexpensive but powerful ELM scan tool & freeware FORScan diagnostic software that's tweaked for Fords, that make for a very good addition for our electronic tool box, here. https://www.ford-trucks.com/forums/1...-scantool.html

We have two coolant temp sensors, one for the dash gauge & one for the engine computer that it uses to help it decide what to do about fuel trim.
It kinda sounds like the thermostat is sticking closed, or reaching a opening point where it sticks, then opens more. So if this is happening you should see signs by monitoring the computer temp sensor. If this seems to be the case, then change the thermostat & a OEM Motorcraft, or a like one with the built in air bypass valve design & Specified operating temp range is recommended, as it'll make bleeding air from the cooling system way easier & its designed to fail in the Open position, which is way less of a problem for the engine in case of failure, because its less damaging to run an engine over cooled than over heated. Don't run a colder than designed for thermostat.
Some beginning thoughts for consideration, let us know what you find.

 

Provided the cooling sysem is properly purged and at the correct level, this suggests the t-stat is sticking and not opening fully until additional engine heat forces it to do so. It is also worth mentioning that thermostat brands other than Motorcraft have also been proven to cause the condition ; as such this is one of those times where using the Motorcraft part can make the difference. The thermostat is a wear item; good preventive maintenance would be to replace it a time or two over the life of the vehicle.

 

i also have to agree that the best place to start is the thermostat. Mine was sticking but it also was causing the upper radiator hose to collapse, then later expand back out to normal later.

I also agree, do not use aftermarket thermostats. I used 2 and both gave me problems. usually had erratic temps with them. Since I put a motorcraft original in, my problems went away.