Spark plug problem

Hi,
My car has become sluggish when accelerating (especially between 1000-2000 rpm), judders when idle and loses speed on hills when driving at higher speeds.
I had this problem recently and my dad changed a spark plug, but as it has happened again I have changed the whole set.
This has improved the car's performance somewhat, but the car still judders a little when idle and it makes a 'flutter' noise when accelerating through lower gears. The ride is much smoother but it now takes a couple of turns of the key in the ignition for the car to start.
Would you have an idea from this description what may be wrong and which next steps to take? I have RAC cover so can get roadside help if needs be, but if there is something I can do or a local mechanic I can take it to that would be great.
The last time this happened I used the higher performance petrol when I next topped up and it made a small difference. I also keep the oil topped up and checked all fluids last week.
Thanks for any advice you can offer!