When were the spark plugs last renewed? Have you used only the correct Bosch ones. Pugs don't like non-genuine spec plugs.
Check for damp on any electrical leads and spray with a moisture dispersant such as WD40.
Check for a hiss at any plastic hoses. They get brittle and can crack with age and with clumsy handling under the bonnet. Replace any cracked hose.
If it persists get the fault codes read.
ChicksFan
November 2013