car has new opti plugs wires coil and runs strong when cold, no miss, after its driven awhile has a very bad miss so bad 6ths gear is pretty much useless becuase itt misses and wont harly pull 6th gear what could it be on a 93 z28

 

My semi-educated guess would be the opti because when mine went it acted that VERY same way.

 

I agree. Did you make sure all of the plug wires are all the way connected to the opti? That is what caused mine to go bad: after a while the terminals got all corroded.