Drop in power on 2ltr 57 plate A3 170bhp

I have an A3 2ltr diesel, love it but it started to drop power then went into limp mode. Took it to the garage I've been using for over 20yrs and eventually thought the problem was solved, unfortunately not. It can be 1500rpm or 3000rpm but it can just drop power and sometimes will stall fully. I have noticed when this happens there is a clicking sound from behind the dash. It sounds like it is coming from drivers side to the right of the steering wheel in the upper dash but no warning lights. I'd heard from another source that they had a similar problem and it turned out to be lose wiring. Is this feasible? Before I attempt to follow and kind of you tube link I can find to remove the dash board. Have already spent £560 on it (inc mot) so any advice gratefully received. 

Thank You