Citroen Xsara Picasso Desire 2 Stereo Controls

[Sheepdisease]

I fitted an aftermarket audio unit and adapter for controls by steering wheel and it all works. However, on the odd occasion when I get in the car the one or more of the controls either ceases to function at all or acts as though it is being held down (and it can be volume, skip, fastforward/rewind etc).

Can anything think what the main culprit is most likely to be? If I start replacing loads of parts it is going to get expensive.