Problem with ECU

I have 2 Z's and the one I'm concerned with is the one I drive every day. It's a 1991 NA with most of the usual goodies and my baby. I figured it was time to upgrade my ECU and ordered one. Put it in and it ran strong up to 100MPH and that was it. Got hold of the supplier and was sent a new computer. Very excited I installed it and it only ran on 3 or 4 cylinders. Pulled it and put my OEM computer in and now it runs like crap. Anyone know what I should do?
E-mail me at: [email protected]

have GMS make you a special chip to run several programs on it.