Intermittently the transmission is not shifting properly

Just to be clear when this happens, you step on the accelerator and the RPMs go up but it is like you are neutral and then it bangs into gear and you take off?

If this is the case, then this is a pressure issue. Basically the circuits inside the transmission are sealed systems so that they can use the pressure from the pump to apply different gears. When there is a leak internally that pressure is not able to build so the transmission, while in gear, acts like it is in neutral. Then the RPMs get high enough to overcome the leak, it applies the gear and the transmission bangs.

This could be due to a fluid issue or a cut seal inside the unit.

If it were me, I would suggest starting with changing the fluid and making sure it is new fluid at the proper level. There could be clutch debris or even metal floating through the unit that is causing issues at random times.

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

Once we know the fluid is correct, the next thing is to check the fluid pressure but we need to do it when this is happening. This is not the easiest thing to do as you need a pressure gauge and install it in the side of the unit.

However, if you are able to do this, you can tape that to the windshield and just drive the van normally and then when it happens, you are all set up to monitor it. Plus you have enough time to see what it does when it is not acting up to see if it looks normal.

This process is below.

If I am correct, then the correction is to rebuild the unit or replace it. Clearly there could be other things but if this was something I was working on, this is what I would be chasing. Thanks