There are only two ways to diagnose a bad ECU, put in a known good one and see if the problems go away, or send them to Roger Bywater (or a specialized ECU repair shop) for testing.
I opted for the first and I was lucky, I got a very good 16cu ECU at a very reasonable price.
I trust there was no actual downshift indicated by the tacho, Rasmus…?
Any sign of smoking from the exhaust when this occurs - ie is the misbehavior due to too much fuel - or too little? Does the engine then recover as rpms increase without further throttle movement?
The throttle pot moves with the throttle - the manifold vacuum drops at opening, the recovers as rpms rise. So the ECU may get the wrong gen from the respective sensors - or the ECU fails to interpret correctly. Take your pick…
Theoretically; this may be an ignition reaction - too much advance for the rpms. A bit far-fetched - the usual reaction is just pinking…
Frank
xj6 85 Sov Europe (UK/NZ)
The ‘could be’ list might get a little long.
Years ago, with my XJS V12, I had a problem where the engine behaved perfectly under very gentle throttle applications but bucked violently otherwise. Long story short, it was the A-bank (inlet) fuel pressure regulator. The cure was to eliminate it entirely…somewhat common in V12 circles.
Cheers
DD