Symptom: At night with the dipped beam on, while braking and slowing down, when the speedometer reaches certain speed (~25 mph), the needle starts to fluctuate up and down instead of just dropping smoothly down towards 0.Over the years, there have been many things wrong with my car, some of which I have investigated and fixed.
I meant to post these findings as a reference in the hope that it may help someone (and myself, because after a long time I might not remember it anymore). It seems that some of the problems are not very common (but some are, I think).
Cause: This is caused by poor grounding and the effect is most obvious when the related electrical circuit loading is high (lights on, brake lights on, cabin fan and A/C running). The relevant grounding points are inside the cabin directly below the A pillars close to the floor level. There is one point on both the nearside and the offside. Note that these are the the grounding for the main harness that runs across behind the dash.
Remedy: Unbolt, clean the points and re-attach grounding bolts.