Help! Where's the Pickup?

Picture this!
91 Dodge Caravan, 160000 miles, 3.3L engine.
When I drive my van sometimes, it seems like it just won't go. No matter how much I press on the accelerator. I'll pull over and turn off the van, if it hadn't killed already, and wait 2-3 secs. After attempting 2-3 times, it'll finally crank. It may do it again and then it may not. If it cranks, it drives ok but hesitantly. It has been driving as if there is no pickup. Also, I don't know if this is related or not, but my door ajar light goes on and off when I hit bumps, and my power locks go click, click, click. Is my van possessed by the devil or what? I've had the plugs, wires, filters, fuel pump changed. I don't know what to do about the clicking locks and door ajar lights. I guess I'll need an exorcist. What is causing all these problems?