Strange problem with Backfiring.

I have just got my car out of the shop having new plugs, and optispark put in and I am still getting what sounds like an exhaust backfire between the shifts. It does it on upshifts and downshifts but only at WOT. On my other car I would get backfires between shifts every now and then when I was spraying because it was too rich. Could this be the problem. Could there be a problem with the Cat? I also have a DTC 18 which was somthing to do with the injectors, I am not sure what to do to get rid of that light, or if that may have anything to do with this issue.

Thanks for the help.

I tried unplugging the MAFS last night to see if that may have any effect, but the problem didn't stop. Although by looking at the readings on my scanmaster it did lean it out some.