1993 Chevrolet Corvette After Running About 20 Minutes Engine Develops Skip On

After running about 20 minutes the engine develops a skip on the passengers side it think (out the passengers side tail pipe) replaced the wires and plugs no difference after 20 minutes same thing.
number 8 plug was fouled and thought that that was the problem it was not. is it the injector gone bad ? also mechanic had replaced a pwb located in or connected to the coil or distr 2 years ago. do you have a clue before I spend a fortune for hit or miss? thank you.