What year, engine size and fuel type?
What happens when it fails to start, does the starter spin the engine as normal or does it sound like it is spinning a bit slower or does nothing happen at all?
Has it been serviced lately, or at all?
Does it start better or worse once the engine is hot, ie after it has been running for 10 minutes or so?
When it is running are any of the dash warning lights on?
Start by checking all the cheap/obvious things.
Make sure the air filter is clean, and there are no leaks in the intake air trunking or vacuum pipes.
Remove and check the spark plugs and HT leads.Make sure there is no water in the spark plug cavity.
spud
September 2009