subaru virgin

My 2.0l non turbo Classic Impreza sportwagon has developed a stutter/hesitation when being driven, had a full service done on her at a local garage whose owner has previously owned a number of impreza's. New oil, air, fuel filters, engine flushed and new oil.Also new spark plugs.
Now the car drives ok below 2000rpm, but over this point until 3250rpm it stutters and coughs then suddenly picks up and goes, very annoying as this is the rev range in which all my every day driving is done.
I have refitted the old sparkplugs and no better, re gapped and re fitted new plugs, fitted a complete set of Subaru plug leads(£50...ouch) at the advice of my local Subaru main dealer(Subaru Kendal) who tells me that a diagnostic check can not be done to pin point the problem, i am very reluctant to purchase a coil pack (£100) until i can prove it is at fault.
Would be gratefull if anyone on here can offer any further ideas or advice on this fault, kind of hoping it will fail completely so it will highlight the fault.