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Code 84 on Vauxhalls is a reduced power/limp mode setting as you already have experianced.. Being extremely intermittent as you have said. it will be hard for any garage or at most myself to place it to one particular component.so long as you understand that...In all ideal situations such problem like this is to leave the car with the garage and hope it acts up with them.whislt the cars plugged into a scanner recording the live data too what occurrs / drops out or stops working, either likely a sensor or fuel presure issue...Have they plugged it in anyway to see if any stored history codes are logged? Probably the most common thing that maybe need to be checked is the EGR Valve and see if that caked or choked up with carbon..Id say that might be the most logical place to start especially on these engines....