Looses power at 3500 rpms

[mtburk1]

I have a 1982 Sport Coupe that runs great. Have added a cone filter and rebuilt the turbo. I recently bought a tach which it didn't have stock. At 3500 rpms the car won't go. It just stops going. I have let off and then it will shift and take off. But in every gear it won't go past 3500. Maybe the tach is wrong but I don't think so, 65 mph is 2200 rpms. The check engine light has come on since it hit 54,000 miles. I have relaced the cap , rotor, plugs, wires and air filter. I got a oxygen sensor but haven't changed it yet. It runs great takes off without hesitaion until it hits 3500 rpms. Anyone have a solution?