the day i bought my 1999 gt I of course "tried her out". Afterwards she started missing in 4 - 5 th gear once I put the gas to it. I ran an Comp. check, and it said I had a misfire in CYL.#6. So, I replaced the plug, and it drove fine. Now, weeks later it does it again, but not nearly as bad; and only when in higher gears, with a load on the motor. Any Suggestions??