Hi Mate,
if you are sure the plugs and leads are ok then it could be an air leak into the manifold, test this by spraying a sniff of easy start around the intake manifold, if the engine picks up then a new gasket is needed.

If this does'nt work then you could possibly have an injector (not delivering enough fuel on tickover) or a wiring problem. The other possibility is that you could be losing compression so best carry out a compression check and see what the readings are across 4 cylinders.

Hope this helps!

Regards, Mike.