1997 Chevy 2500 4x4 - Loss of power

1997 Chevy 2500 4x4, 350 Engine, Automatic Transmission. Runs great for 2 Hours and then won't hardly go. Still idles fine, still runs but without pulling power. Fuel Trim is within spec, O2 sensors are new and working correctly, Back pressure on exhaust is 1.0 (within spec) Fuel pressure is 65 KOEO and 58 KOEO. Every thing seems to be in order. But when this happens you can spray some starting fluid in it and get it to break the tires loose as long as you keep spraying in starting fluid. I was wondering if anyone out there has seen this and if it could be related to the spider injectors ???