As mentioned by Nigel, have the diagnostic codes read (OBD Scanners are available on eBay & are a worthwhile investment). You don't mention the vehicle age (could have nikasil lining issues like my xjr!), but coil packs are often suspect too.
Paul Jones (Medical Electronics Engineer - Ex TV/Video/Audio))
September 2009
Hi Richard, have the car serviced and the diagnostic codes read, this will point out if any valve or sensor not covered by normal servicing is faulty/intermittant
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Try fixfault.com to fix cars.