Problem with Mercedes Sprinter - Electrical?

I have a real strange problem with my Merc. Its a 311 CDi 2003 Luton on a 53 plate. It is serviced every 6000 miles (part then full) and it has 216k on the clock. Recently there have been problems with starting and loosing power. Sometimes when you are driving and put your foot down to overtake etc, all power goes, then after a second or 3 it comes back. It has been in and out of the garage for ages - its been tested where they have replaced sensors and all sorts - but still the same. Its been tested on the computer and there are no fault codes - the garage thinks it may be an ecu problem? I would love to find someone to give me some solutions. I am in Middlesbrough