i have a 2003 impreza wrx prodrive, when im in 3rd gear and get to to 4k revs the car starts to judder back and forth. it's like someone is tapping on the break,only does it in 3rd and over 4k revs, would appreciate any help

Hi 

have you recently had a new clutch ?

 

does this happen when car is warm or cold 

no clutch hasn't been changed this year and it does it whether it's warm or cold

Sounds like maybe a seal has gone and there’s contamination 

or maybe clutch is on way out 

Does it do it at light throttle or just half / full throttle on boost ?

Any mods induction kit , intake pipe , cat removal ect .

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full throttle and the only mod is the prodrive kit

Is it like a violent jerking back and forth while not picking up much or any speed? Does it backfire like crazy at this point to?

Sounds like a misfire , maybe weak spark being blown out by the boost .

When was the last time it had a spark plug change ,

Whip the plugs out, see if there's any particular cylinder that seems blacker than the others to indicate a misfire. Also check the insluation on the coilpacks and "pigtail's"

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