I was wondering in what region you lived. Perhaps there were speed restrictions applied via the ECU firmware. If the car is running properly it should still be accelerating quite strongly at 150KPH. Does it stop accelerating quite suddenly at 4000RPM?

It is quite easy to build a code reader for the 16 pin OBD. Only a few basic electronic parts are required - a 12 volt LED, pushbutton switch, and a few banana plug leads. These are easily available in the U.S., though cannot speak for your locale. You can find the instructions, wiring diagram, and code lists in old posts here. Suggest searching using the terms "Arthur Dalton code reader" and go from there.