My 1994 z28 just had a major electrical tune up. I replaced the opti, new plugs, wires and even a new waterpump. It now runs rough untill 3000 rpm and then it runs great . I just spent a fortune and really don't to take it to an expert yet. It has 107,000 miles and no codes appear on a scanner and the check engine light doesn't appear .........what could be the problem ?

Replaced your fuel filter recently? My friend's C4 had a similar problem and that fixed it right up.