What can cause stalling at high speeds?

The van starts and idles fine and shifts fine while driving. At low speeds it will get up and go without a problem except until it gets around 40mph or higher. I can be accelerating or holding the gas steady anywhere from 40mph or higher and it will be fine then it starts stalling almost as if it's misfiring.

If I check the speedometer while holding the gas steady the needle starts to drop showing I'm losing power. If I push down on the gas the transmission kicks into overdrive to compensate but once I let off it will stall out again. Sometimes I can push on the gas and let off a few times for it to hold power until it stalls out again. If I drop below 40 though in areas that are 35mph speed limit, it runs perfectly fine. Also the check engine light does not come on while it stalls.

What areas should I be looking into first?