Could just be corroded connector on the back of instrument cluster, as it heats up resistance goes up and it loses signal.
There are 3 connectors on the back
C214, C215, C216
Pin 1 on C215 is the speed signal, grey/black stripe wire
But could be other things as well, like the speedo/odo cluster itself, I would check around to see if you can find a replacement, don't get it yet but just find it in case you need it.
Speed signal is an AC voltage signal, only way to test wiring is with signal from VSS, and it generates it own AC voltage........
So only way to test it is to get rear wheels turning so VSS generates some voltage, i.e. jack up the back end and put it in gear.
And since it takes 15-20min for problem to show up.........that would make testing tougher.
The 1990-1997 Rangers used a gear driven VSS, like this:
http://www.therangerstation.com/tech_library/images/speedsensor.gif
These were used with speedometer cables, AND had VSS for cruise and computer, 2 wire plug in
If you could pick one of these up at a wrecking yard, pretty much any Ford from early '90s would use this, then you could use a variable speed drill to generate VSS AC voltage to test speedo/odo