Ford Focus - erratic rev counter

I have a 05 ford focus with 60k miles on the clock, and in the last month the rev counter has occasionally dropped erratically when in idle or first gear and has in the past feels like it misfires or loses power whilst driving.

Can anyone please let me know if this is a common fault and expensive to sort out.

I have had new ignition coils in the past year and major service last year.