Van runs great until it heats up. generally about 15-20 min.

Maybe gear box heating up would check fluid level

Probly no one ever changed the trans fluid,now you did and its to late,But get a bottle of Lucas trans slip.cost about 12$ at walmart,put the whole bottle in the trans,it is really thick, so heat the bottle on a register for a good while so that it will pour right in in this cold wheather.Let car run for 20 min. than try driveing it.if it wont move,let it rin another 20 min. if it moves drive it around untill it starts driveing better ,might take up to 3 days to get moved through trans real good.It worked for me on 6 cars now

If it was me, the first step would be taking it to a transmission shop for a tranny-fluid change (unless you are a real wrench and can do it yourself). The transmission fluid can get dirty, breakdown from heat and/or general use/abuse, and the filter can become clogged.

Okay you will need to program the shift points again this is normal when battery voltage is lost, you will need to shift yourself and try to get it to shift when you want it to, what you do is remove the negitive battery terminal for 10 mins, then connect it back, start the van when you take off the shifter must be in low gear try to go slow until you have a straight away then accel and shift gears up until you hit the last drive gear make sure to shift as it use to or as close, dont rev to high before shifting as you shift up the computer stores the info and thats the way it will shift if you make a mistake unhook batttery again and try it again. i cant rememeber the exact shift points but i think you will know as its the van you drive, doesnt have to be perfect, for example start off in low and when you hit 15mph shift up a gear then ithink at 25 shift up and then 35 or 40mph shift and so on, you dont want the engine to rev to high between shifts it should be smooth but you you feel it a bit, hope this is helpful. let me know the out come, i do this all the time, dodge ,chrysler cars and vans. good day.

Hi!!
Check the transmission fluid level, here is how.
Engine running, move shifter one gear at the time until you get to one and then back to park, do this a couple of times and then check your fluid level. Also check for burnt smell on the fluid. Fluid should be of a pinkish color, if too dark, service transmission.
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how about putting the van in drive with one foot down hard on the brake and rev the engine, does it smooth out now? if it smooths out, i would pull the air cleaner tube to the throttle body and spray the throttle body with carb cleaner, be careful if using the red tube, I have had it blow off before into the intake, not good, open the accelerator and spray it out really good, it will to start until all the cleaner clears out.

Try removing the idle speed motor on side of throttle body, and spray it clean, as well as the port it came out of, with throttle body spray cleaner. Don't pull and extend shaft of idle motor, just leave it in position it's in, and make sure you don't lose small "O" ring on idle motor. Re-install and try it. If it cures it for a while, then starts to do it again, replace the idle speed motor.

check ignition timing

It could be a spark plug problem..I would remove a few of them to see what they look like. It may not be firing very well when it gets hot, and new spark plugs are never a real bad investment, since they're generally very cheap. Not sure on this, but it's a good place to check.

change plugs