Variations of speed on a constant slight throttle

[CERBY8]

I have a 2008 focus st 15000 miles on the clock and it sort of surges on a constant throttle very slightly up in speed. Also you can either accelerate slowly where it gives you quarter boost or give it full throttle but not much inbetween. If you half throttle then put your foot to the floor it will not give full boost and often misses a beat and doesnt pick up cleanly. Any thoughts?