'96 GMC problem with random mis-fire

I'm having problems with a '96 GMC Sierra with the V-6 engine and manual transmission. The truck has 97,000 miles on it.

The truck starts missing at highway speed causing the check engine light to come on with a code of random mis-fire....I'm not sure of the number. It doesn't do this all the time. It seems to do this more when I'm really on the gas around 60-70mph.

So far, we've tried a regular tune-up (plug wires, plugs, distibutor cap, rotor, fuel filter, etc.) and have checked the fuel pressure to be at 60 psi. During the tune-up, it was discovered that the wrong spark plugs were in the truck and were not "hot" enough. They were all covered heavily with carbon. Since the tune-up, I have been burning the 93 octane with injector cleaner added to it for the past two fill-ups but nothing has changed. I can pretty much make the check engine light come on when I want to. My mechanic, who I've always gotten good service out of, has told me to run the 93 octane with injector cleaner added to it for a month or so to see if the problem clears up.

Has anyone had this problem or does anyone have any suggestions?

UPDATE: I've run about 6 full tanks of 93 octane and fuel injector cleaner through the truck and the problem has went away. I was never a believer in fuel injector cleaner but I am now.