Hi, I'm new here but not new to working on vehicles. I have a 1999 chevy Silverado with 5.3 vortec ...the problem: engine has poor performance (sometimes) almost like its running on four cylinders mash hard on throttle and it thinks about it then starts pulling suddenly? The cure: when it starts running like this I can pull over, shut the engine off, get out and unscrew gas cap then reinstall it and start truck and usually it will run fine? Leaving truck running and removing gas cap doesn't work or just shutting off truck doesn't work. Truck always starts, idles fine, fuel millage isn't bad. I have replaced all normal stuff wires, cap, plugs, coil, thermostat sensor, Gas cap, new fuel pump and filter? Any input? do not have a scan tool nor have I tried one.