Stalled out and will not start

I just bought this vehicle and I suspect that it has been sitting for a while. I drove it this morning and it drove fine, though it did have some hard and delayed shifts. This afternoon I was driving it on the highway at 65 mph (made it about three miles) and had a drop in power. I tried stepping on the accelerator further and it bogged down even more. I pulled into a parking lot and it died on me and won't turn back over. There were no lights on on the dash when this happened. Coolant temperature was fine and the battery had a good charge. Enough to crank it a few times to try to get it to restart. Any idea what it might be? My gut says bad fuel, but the PO says it is only 6-8 months old. It drove fine earlier, so I'm at a loss.
Thanks!