Does anyone know what causes this? It is not overheating and after a few minutes I will lose power and engine runs erratically. If I shut engine off for a few minutes, it returns to normal but for how long I never know.

 

How do you know it's not overheating?
You lose power because your van was in LIMP mode.
Do you have a scan tool so you can read the temperature?
What you can do easily is to unplug clean the electrical connector on your temperature sensors (don't remove the sensor itself). You have 2, one for gauge the other for computer (called ECT). Clean them with contact cleaner and use wire brush if necessary, then get the reading again. The ECT is more important and if it gives wrong data to the ECU to cause LIMP mode when it's bad or just bad connection, your best scenario.
Another thing is to ensure your coolant level is good, and check for leak.

 

Where are the sensors located?

 

Check my signature below, what's the engine size? It might not matter but for me to look up the diagram engine_size is a required field.

 

1999 Ford E250 4.2L 2WD AUTO

 

As you can see above, engine size matters and 4.2L is not no.1 but no.10 above.

Below is the one for the gauge.

Find below #14 (for gauge), #15 (for pcm):


Below is the zoom out view where C137 (for gauge), C142 = ECT (for pcm)
(for 1998 e150 4.2L, I can't locate any good diagram from your 99 e250 4.2L but location wise they are the same)



As I mentioned previously your best scenario is issue with your sensor(s), just clean the electrical connectors on both sensors and inspect the wiring, but more than likely it's your thermostat that's going out. Why?
Because of the LIMP mode. Your PCM received the signal from C142 (ECT), you, at the same time, saw C137 (for gauge) the quick rise of temperature. For both sensors to go bad at the same time, the chance is almost next to zero. Anyway, just clean them, warm up and take short trip only.
Also, why not replacing the ECT sensor? 70% of the time it's the issue of the connector, the wiring, the sensor itself has a long lifespan perhaps as long as lifetime of your van. If you think you want to replace it, go ahead it's dirt cheap anyway. But be expected during removal some coolant is going to spill out and get ready to catch those with a towel. Below is the torque spec if you decide to replace it. Same spec for C137.

 

Thanks for the info. Much appreciated.

 

Find below 2 pdfs: Pinpoint A (Loss of engine coolant) and Pinpoint B (Engine overheats)

 

If after you do all the things suggested and still have the problem, please get back to us, thanks.