if it is a sticky valve or lifter you can generally pick it as a ticking/tapping sound coming from the rocker cover , give the car a bit of a rev with the bonnet up and you can usually pick it . any decent quality injector cleaner will do the job , i think cem cleanpower is probably about the best of the bunch . it's supposed to clean the whole system . i would keep an eye on the fuel filter and if it seems to be blocking change it . come to think of of it , i have come across fuel filters causing a similar problem . if your filter is getting near changing maybe change it first and then add the injector cleaner and keep another spare fuel filter just in case it is required .