Help needed please with my Yamaha jog rr

Hi all.

I own a Yamaha jog rr 50cc.
The bike starts and ride fine but as soon as the bike hits 30mph whilst full throttle it loses power and gains power all under full throttle and won't go faster than 30 but as soon as there's a slight downhill my bike will get speed again and go to it's usual 45mph. But eventually i it will die back down to 30 mph again whilst full throttle.

Can anyone help me sort this out please ?