2006 Volkswagen Touran intermittent sluggish start, pumping of accelerator needed to start

Problem started in mid summer, so not a cold starting problem. On turning ignition key, engine attempts to turn over and immediately cuts out. Only way to start engine is to pump accelerator vigorously, and when it starts a cloud of dust/smoke is emitted.
Local garage did diagnostic which came up with manifold seal, cam belt sensor and glow plugs. Car went in for major service and cam belt change so hoped they would fix problem whilst engine was apart. Cam belt sensor had no fault so was not replaced. Some corroded wiring was fixed, as was manifold seal. Fault log was cleared. Engine runs better due to service. Was happy for two days and the problem happened again. Does not happen every day/time car is started, can go up to a week without happening. Car has been back to garage twice and they can not locate problem, or it does not happen in the garage. When car has started (after problem) it will start over and over again throughout the day with no hesitation or problem. There are no warning lights on the dash display. Allowing the lights to go out on the dash before starting doesn't always work.

Please help, has anyone else had this problem???