Last week my 96 Z-28 started missing badly, codes 300, 301, 303, 306, etc (misfires), along with a raw fuel smell. Mt first fear was the Opti. Took it to a mechanic who ran some diagnostics and came to the same conclusion - PROBABLY the Opti, possibly plugs and/or wires. The coil checked out OK. So, I limped home, knowing I'd have to drive like that for a couple of days, waiting for the weekend to come so I could get to work on it. A couple of days later I was low on fuel, filled it with Premium, as usual. After about 10 minutes of driving - SURPRISE, no stumbling, no more misfires, it ran smooth as silk. Turns out the whole problem was just a tank full of bad gas. I've had issues before with bad fuel causing a slight stumble, but never this bad. Turns out this particular motor, much like it's owner, has occassional bad gas issues, some worse than others. So, just an FYI, this is something you'll want to check before diving into the mechanical fixes.

 

Or,your water pump is gettin ready to go.It will leak directly over the opti,causing it to spit and sputter,untill it dries out.Are you loosing any coolant?Hope this helps.(Also,if you get the opti wet,it takes a few miles to dry out.)