Silverado misfiring on road-Idles great.

[Wartrace]

2003 4.3lv6 engine. Problems started when I ran the vehicle low on fuel, the low fuel light was activated. To compound that, the driveway is about a 30 degree down angle when I park the truck to lock my gate. I left the truck running.
I replaced the fuel filter at the first sign of trouble. I have run a full tank of gas through it with the high end gum-out product. I have also cleaned the throttle body with cleaner (Teflon safe).
It idles fine. It had a hard start problem when hot before the cleaning- now it starts up hot or cold.
The problems don't start until It has run for 10 minutes or more. Its as if the engine cuts off briefly while at highway speeds. If I mash the gas it accelerates great- let off and it starts missing again. No problem climbing hills at all. I had it up to 90mph today on the interstate today and it didn't miss a beat. When I am cruising at 70 it stumbles.
Any ideas?