the pc was initially built by a friend as an exercise for him as he was ill with cancer.
He is a professional hardware it person for some 30 yrs, since spring actuated adding machines even!
the machine has been running for 18mnths so not any of the items refered to in previous answer.
I ended up handing it over to a PDC repair shop here that I trust and is very good. The owner actually wrote the warranty for HP.
hes diagnosed random fault in the video card after much testing and poking.
thanks for help.