I have a 1989 5.0tpi iroc z. I have had vacuum line problems before and thought this was another one of them. Car idles a little high at times. when under load of any kind it acts like it is misfiring. all plugs and wires are new and distributor, cap, and rotor new. when I hit a bump it will sometimes correct the problem till I hit another bump. All vacuum lines have been checked and replaced. I have no engine codes at this time. and the gaskets at the plenum and runners have been replaced to ensure proper vacuum. I have exhausted all my options. the thing that puzzles me is the fact its not all the time and i can hit a bump in the road and this will trigger it. thought possibly loose wiring but i don/t know where to start. Any help is appreciated!!!