Aprilia rs 125! Seems to be struggling at 5 -6thous revs then it shoots off?

Right i have just got my aprilia rs 125 i warm it up properly but then when i want to give it some beans it pulls off slow to about 5000 revs and then between 5000 - 6000 is just sits for about 3 - 4 secs then it just takes off what is this? why is it doing this? can i get rid of this?

another question i have noticed that when i change down gears is seems to be cutting out when i bring thre clutch in? any ideas why its doing this and sometimes it doesnt want to start and i have to use some revs and if i stop reving it cuts out any ideas''

thanks really need the advice first time with an aprilia!