it sounds like the coil pack is breaking up, if you pull the plug leads one by one you should notice the engine running worse on 3 of the cylinders and just the same on the bad one or the bad pair, then fit one of your old plugs to the bad cylinder/s lead and earth it (by touching the engine with the metal body of the plug) and check for a spark no spark bad coil or engine management fault (very unlikely), use well insulated pliers and/or thick work gloves when working on the ht side of the ignition circuit it will give you quite a shock , you could also check the condition of the wires feeding the coil i have seen a couple that have broken one and only run on 2 cylinders