I'd GUESS insufficient fuel pressure too. But instead of guessing, I'd do as Jerry says and actually measure it. This will require a pressure gauge. A combination vacuum and fuel pressure gauge is your best bet and can be had for under $20 at Harbor Freight and may be available for loan out at your FLAPS. You may need a longer hose to reach into the cab so you can measure while it is failing. Check your Chevy Maintenance Manual for proper readings.

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.