Whe accelerating under heavy load ie in 3rd 4th or 5th, my turbo spools up, then the boost seems to die and then come back in a series of waves making the car lurch forward at intervals. I thought it was down to some dodgy fuel as it happened a few months back when i filled up at a "Total" service station on regular RON fuel. After refilling with Shell opitmax the problem went, but it has now returned. I ran the tank dry then put an octane booster and Optimax in. It is still lurching though. Any ideas?