Ca18et runs bad/missfire

[Westerlund]

My s12 with ca18et has started to run really bad and missfire up to around 3250-3500rpm (dont know how exact the oem tach is)
It misses and spits up to that RPM range but when i hit 3500rpm mark it's like someone turns off a switch and the car runs great and clean from 3500 and upwards.

I have changed the plugs and coils/power transistors (to used ones) without any luck.
Any Good ideas on what can cause the issues?
For me it seems to be a signal issue due to the exact rew range it always run bad in and switches to run Good att the exact RPM every time.

I have read about transistor failures, that the car starts du run bad when the boost starts to come but for me it runs good on boost.