Right. Firstly it probably has no distributor, and you are prehaps refering to the coil pack? these never go wrong and are bullet proof unless pysically broken or cracked by being hit by something. NEVER change just the plugs, always change both the leads and the plugs as you will not know which are at fault. If this does not cure it then i suspect it may be "jacking up" the lifters, sometimes refered to wrongly as sticky valves. When ford designed the engine they fucked up, and subsequently invented a thinner oil (5w30) to stop the hydraulic lifters from jacking up when cold, this may not be your problem as this is WORSE at higher rpms, not better, If it had this problem then an oil change to the correct oil will cure, or certainly make it better. Failing that you have an engine management problem, either coolant temp sensor or IAT sensor, throttle pot or map/maf sensor is at fault, or the related wiring to such devices is faulty. Diagnosing a problem like this without seeing the car is near impossible, especially over the web where conflicting and sometimes incorrect advice is given.