2005 Volkswagen Sharan problem starting.

The problem arises intermittently irrespective of whether the engine is cold or hot. On starting the engine turnover note sounds totally different from normal. Sometimes it catches and the engine starts, sometimes it doesn't start and it can take 20 or more times before it fires.It then frequently runs briefly at roughly half power, often cutting out and stalling, which seems unusual for an automatic. Often following on from the strange engine turnover noise start it will cut out after running for a quarter mile or so making driving a potential problem. There is no visible smoke, which might suggest turbo or injector problems. My mechanic doesn't seem to know either. Any advice would be welcome or anyone had a similar problem?