I just bought a 96 z28 with 70,000 miles. i dont know what kinda maintaince was done on the car put it has a weird idle. It will idle fine went you first come to a stop light but only for like 5 seconds then it starts getting kinda ruff the rpms go from like 600 to 800 not really volient or any thing but when i look at the battery gauge it goes down a little with the rpms (and the lights in the dash dim a very small amount). I put some injector cleaner in it and i was going to change the fuel filter. What else to try? Its to soon for plugs right?

 

Try cleaning out your throttle body. Most importantly the little passages that let air in when the throttle is closed.