I have a 94 855 turbo. when the rpm hit around 3000 to 4000, it seems don't want to go any higher and engine sounds like one of the cylinder not working and the rpm will raise very slow as well as the speed. I also notice the idle is not stable when at red light (not noticeable by looking at the rpm but I can feel the jerk). It is more noticeable when a/c is on. Everything had been check incl leak, plugs and so on but nothing found. Fuel filter has been changed and throttle body has been clean. My mechanic suggests it could be a plug cat. It all happen after I shorten the wastegate and even restore it to the orig setting doesn't change except the problem might delay to 4000 rpm. Other than that the car has lots of power and derives very smooth if I take it easy and keep running in low rpm. I have search all the forum I know and noone seems to have this problem. Also this is very hard for me to explain, hope you guys understand what I am trying to say. Thanks.