hi recently I've got problems with my Saab besitating a lot on all gears. I've replaced the coil, and now it drives like a beast. Fast as never before. Pulls like crazy on 1,2,3 gears up to 150km/h. Then when I put 4th gear it hesitates/jumps. It was like this since I got him, even before the coil went absoliutely bad. Not sure if the new one is damaged too, the idle is ok, it pulls nicely on 1,2,3. I'm thinking maybe the pressure of fuel or blow off valve or some turbo piping. But why only on 4th gear around 150km/h. All the hoses have been replaced.

As the background the car has stage1, with intercoole (550x230x65mm, fmic advertised up to 500hp) and 3" downpipe.