intermittent kangerooing and loss of power?

Have had the van in to slaters garage several times with a cost of £800 so far. They have updated the managment system,serviced it,changed 1 injector and done a pressure test.
The van can drive 2 miles or 100 miles befor the problem occures .Suddenly it will start to kangeroo and white smoke comes from the exhaust. Can still be driven slowly on the bite of the excelerator . If i park up and go back to it the van will drive fine until the next time. Nothing shows up on the diagnostics in the memory and the garage says they have no idea ,just seems they keep replacing parts in the hope that it solves the problem. Deperate,thanks