engine violently shakes at mid rev range

Hi,my partners Car has recently started to violently shake when accelerating and reaching 2500 revs.the car is fine below them revs but has happened in gears 1-4 and only seems to happen when the revs reach the mid range.we've had the ecu checked and it's come back fine,its been serviced within the last week and has had a new crank sensor fitted too.the mechanic is stumped as he can't find anything mechanically wrong with the car.was hoping that someone might have come across something like this before as I don't want to plough money into the car unnecessarily.I'm leaning towards it being electrical but I'm not sure,any help would be appreciated.Thankyou.