ROVER 416 rev problems ?!?

I need your help with my Rover 416 P-reg. It has an idle tickover at 1500rev (initial start up) and has an 'over-rev' problem when Im driving. It happens when I need extra power to overtake or when I change up a gear e.g. @ 50mph from 4th into 5th it went up to 5500rev then back to normal again. Its almost like it sticks, over revs then returns to normal. Sorry if I dont know the correct terms but Im trying to explain it as I experience it.

Any Help please?