Uneven velocity response on Virus TI

Hi,

we discovered yesterday that on our Virus TI (1), there are three keys responding differently to playing velocity than others.
They react much stronger - making sound programs that use velocity difficult ("dangerous") to play.
Mechanically, they "feel" the same as the other keys, they just seem to translate the velocity in a different way.
Since the problem is shared between sound programs, it is to be assumed that this is a hardware/calibration problem.

(The keys are distributed over the keyboard G#2, F#3 and C4 counting from the lowest key on the 61-note keyboard as C1)

What could the cause be and how would one go about fixing this?

Thanks so much for the reply Jörg.

I did end up having success fixing the problem this evening.
As you suggested via email, the problem seems to have been the keyboard bed.

I carefully unscrewed the virus, removed the keyboard, disassembled it and cleaned all parts of it - using alcohol where necessary.

I discovered that the two long green circuit boards on the back of the keyboard (which carry the rubber contacts) where full of black soot particles if unknown origin.
I am assuming that this dirt might have created unwanted electrical contacts and changed the resistance/impedance.

What I used to clean it off was mostly kitchen wipes, sometimes with a little alcohol.
I tried to stay away form the black contacts that are touched by the rubber keys.
I did remove and reattach the rubbers however in order to make sure there were no particles left underneath.
(btw - Re-attaching the rubber elements works with a thin metal piece, like a nail, best with flattened tip).

Summarizing, it seems the problem of uneven velocity response in our case was caused by:
Dirt on the electrical circuit in the keyboard bed.
Not under the rubber switches themselves, but around the soldering contacts on the circuit.

The work I had to do to clean it up was pretty extensive, and of course doing this there is never a guarantee that one doesn't end up doing more harm than good,
so I can probably recommend leaving this kind of repair to an official workshop.

Really glad that it works again now though.