Tranmission?

Hi. I am having some problem with my Nissan March/Micra Ak12. It is feeling a bit more sluggish thesedays. When i accelerate (press the accelerator) the rpm dials doesnt rise as quickly as before, and of course consequently the speed. This happens mostly in high gears from 3 to 4 or 4 to 5. Lower gears accelerate fine. Sometimes when accelerating at high speeds the rpm blips, i mean i revs up high and then drops suddenly before stabilizing a bit higher up. As i said these mostly occur under highway dreiving conditions.
I this a problem with my transmission or my clutch plate is slipping?