I just went through a similar problem with my 97ss.
I changed many sensors and optispark only for it to run the same, like crap.
It ended up being a header exhaust leak. So check your gaskets or any leak before the 02 sensors. If your sensors are picking up too much oxygen they will make the car run rich and retard the timing .
Take off your headers or exhaust manifolds if your car is unmolested Replace wight new gaskets and see how it runs.
Worked for me hope i could help.