Why is my1992 chevy Astrovan shifting hard and bogged down when warm?

Starts fine and idles a little high. acts fine until it reaches highway speed then it starts to chug and act like its short of fuel. I would guess its the fuel pump or filter but it also shifts at different speeds then when cold and I'm almost positive that I saw the voltage meter spike and jump a few times. I just bought the van and dont know much about chevy. I'm wondering if its fuel problem or wiring and electrical, or even the transmission or combination of them. Its an automatic transmission. Also I noticed that its a little high on tranny fluid and am told its really bad for it, so maybe its been that way and messed up the tranny.