Knocking/Missing after warming up

My 94 Z28 only has 47,000 miles on it. After running for 10 or 15 minutes it starts to miss and knock badly. i can still drive it, but i can barely give in any gas. I know this could be an opti-spark problem, i just wanted to ask if there is anything else that could be causing this.

Thanks for everyones time.

Without a scanner hooked to it you are just shooting in the dark.
The cheapest part to replace that could cause that problem is the 02 sensor.

ok thanks morley. Anyone else have any other suggestions?

hmm... bad distributor, something with spark plugs or wires or cap and rotor. possibly MAF sensor. theres lots of things