When my 1998 plymouth grand voyager runs a long time the transmission starts to have a hard time shifting or not shifting at all until it cools down.

It sounds like you need to have your transmission fluid flushed. As transmission fluid ages it starts to brake down causing improper shifting and less protection for your transmission against friction and heat. I think if you have your transmission flushed it should take care of your problem. I hope this helps you out.

I would recommend getting the fluid and filter changed. As the transmission warms up the fluid gets thinner. if at high temps the transmission isn't operating properly your fluid is probably not operating efficiently

How many miles? I'm guessing 150k atleast. A complete transmission flush/pan filter is probably your only option. Other than that, it is mechanical and needs to be rebuilt or the torque converter replaced, etc. Basically, if the flush/pan filter doesn't work, you probably need a new vehicle.