1990 Chevy Astro problem with my van

My van has been running fine over shorter drives. But this last road trip (3days long), I've found that the engine is running much hotter than normal after the 1st day of the trip. But when I’ve been driving it for a longer period of time (say for 4 hours+ straight) when I stop the van (at a stop sign for example) the engine sounds like it's about to stall. When sitting in drive but with the brake on it sounds like its about to stall, then revs a lot (also does this in park). And does that until I drive. When driving it, at any speed (whether it be 35 or 55 or whether I’m accelerating or not) it feels like someone is hitting the accelerator and releasing it over and over for 2 to 3 seconds before it can continue to maintain speed. The problem seems to go away after I’ve left the car to sit overnight and runs fine again the next day, until I hit about the 4th hour of driving.
I’m not sure what this could be? It’s a pretty persistent problem but is only visible when I’ve been driving it over long distances.