.....If I drive for more than about 20 to 25 minutes at a stretch, my
van starts to lose power, almost like it's overheating. .........

This sounds much like 'Vanagon Syndrome' , associated with the AFM and a
confused ECU. When this happens try pulling over and shutting off the
ignition but only wait a minute or less, then try to restart.  If the
engine then runs normally you are probably dealing with the Syndrome.
There could be  other problems (already covered by other posts).