As above, had a similar issue, traced it to the wiring in the engine bay on the bulkhead where the loom connects (there are 3 multiplugs i think). Anyway my SECS monitor showed that the ECU was reading double the actual RPM, and hence the ECU "thought" it was hitting the limiter all the time when it was only hitting about 3500 rpm. Give the wires a wiggle and see if it cures the rpm counter fault, maybe a biut crude but atleast it'll give you an indea where to start, and it's a really common problem.