Speed density and NGK TR6's?

I recently put a set of TR6's (gapped at .035") in my '93 Speed Density Z with the intentions of running the bottle. After only a week of running them, I developed a miss. So far, I've pulled the #1, and # 4 plugs and they're both black. I've got a new set ordered, so I'll see how it acts when I get them in.
Could it be that the speed density system doesn't like plugs this cold? Anyone else with a '93 running TR6's?

Frank

I ran them for years with the stock computer - I'm currently running an autolite 1 step colder (equiv to a TR7)