A bit more info would help.
Petrol or diesel?
Is it worse when engine is hot?
Worse under load, eg accelerating up a hill?
Any "performance" parts fitted?
Check all air, vacuum and breather hoses for leaks or splits.
Has it been serviced recently/ever?
If not I'd fit new plugs and leads (or coilpack if it doesn't use leads).
doofer, September 2009
