1991 GMC Sierra runs rich, wont idle

My GMC will almost die and then rev up, and miss badly on acceleration. I have tried to troubleshoot it and have not made any progress. It does not bring up any codes, I have changed fuel filter, plugs, wires, rotor, dist cap, o2 sensor, coolant temp sensor, throttle pos sensor, idle speed module, intake manifold gasket, rebuilt throttle body, replaced injectors, and fuel pump. When it runs the intake manifold gets alot of excess fuel sitting in the bottom. I have taken the truck to different shops and they are at a loss. Anything would help, Thanks