I bought a used 2002 GT convertible with 16K miles about a month ago from a dealer completely stock. Added the C&L 85mm MAF housing and K&N filter combo, then had the Steeda underdrive pulleys installed (bought from stangnet here, and yes, I believe they're the correct ones). I've disconnected the battery four times, each time for a minimum of 15 minutes to reset the computer since the mods were installed. Since then, I have been getting an intermittent engine misfire according to the computer codes. I pulled the MAF and used a spray electronic degreaser to clean the wires in case any of the air filter's oil was somehow fouling them. I've taken it to the dealer twice now. The first time, they cleared the computer and said that they don't recommend underdrive pulleys. The second time, I had them check the torque on the balancer pulley as I've read that an incorrect setting can cause misfiring. The torque is correct according to the mechanic. They did come up with some Ford technical bulletin that states that underdrive pulleys do cause sporadic engine misfire. Big surprise there. I've driven the car a total of 10 miles since getting it back from the dealer the second time. The SES light is back on full-time, and this morning, it blinked at me two different times, then settled back to a steady burn. I could feel the engine's rough response when the light was blinking, so I'm guessing it was actively misfiring at those times, then smoothed out when the misfiring stopped. From what I've read and been told, both of these mods are pretty well tested and very safe and stable. My experience with them has been lousy to date. I really like the extra kick I get from the throttle, but I am not going to drive it forever with the engine misfire. In fact, the mustang's in the driveway until I can figure something out to try next. If I have to, I'll remove the pulleys and the MAF/Filter upgrade. However, I'm hoping someone out there can suggest something else to try to get rid of this problem once and for all without sacrificing the mods. I have yet to re-install the original air filter and MAF housing. Why? One of the torx bolts was thread-lok-ed into the brass insert in the plastic housing and pulled the insert out of the housing when I removed it. Of course I dropped the insert and it rolled away somewhere where I can't find it. I'm working on a replacement insert, but they're a **** to get in the correct size (Metric??? Isn't this an American car?). If/when I can secure the MAF with both bolts, I'll try re-installing the stock components. What's the concensus out there? Do some cars simply just not like these mods? Do I need some undocumented computer setting change? Thanks for any help y'all can provide.