Intermittent faults are always difficult. Time, patience, and perseverence are your best friend.
My feeling would be to concentrate on the fault reporting system and try to analyse how faults may be generated.
I am wondering if there is maybe a faulty plug/socket/ contact problem and plugging/unplugging may correct the fault temporarily. I suggest you keep monitoring the fault codes to see what is being reported.
Unfortunately if you have to pay someone to do this you could be up for some $$$'s
good luck with it.