If you feel comfortable running it up to 6k anyway, how about taking to to a rolling road such as Track n Road to see if they can investigate the problem? My sigma academy car had an intermittent misfire at 5k that was proving very difficult to find so i took it to the two Steves. Took them an hour or so on their rig but eventually they found the problem. Its a round trip of 400 miles for me to trailer the car over but well worth it and we couldn't find the problem locally - it turns out that the bolt holding the earth lead (engine mount to chassis) was very slightly lose and causing the problem. Cleaned and tighted and all sorted. The other earth leads were fine including the main battery to bellhousing one that you'd think would be all the car needs. They only charged me for a couple of hours work so not a full RR setup and it was fitted between a couple of other jobs.