fault code P0236 overboost valve

Hi Folks. Bought a Multipla jtd 110 with a running fault. 2001, 70k miles, previous owner done 5000 miles in 5 years to and from Tescos in London. no power, changed MAF sensor and power a lot better but rattles on acceleration almost like pinking on petrol engine ( I think that might be an injector stuck open a fraction).
stripped and cleaned EGR valve.
hard driving will bring on fault light. FiatECUscan reveals P0236 code overboost low volts.
Driving with laptop connected says that the boost pressure is half of the desired amount.
At a standstill and revving it up produces BROWN exhaust smoke!!! (I'm using injector cleaner in this tankful to try and free injector, it is getting quieter)
Could the overboost solenoid cause all these problems?
thanks folks Gez