sukida sportman 2 50cc not running right?

Hi
Right were to start lol

Things i have all ready done : New plug , New fuel lines n filter , Taken the carb to bits cleaned it

It starts fine , Ticks over fine , When i set off all is good i get to about 20 mph and it starts to lose power , Its like its not geting fuel but it is , When it starts to lose power its like i have run out of fuel so i turn it off n start it again all is fine untill i hit about 20mph and it does it all over again , Also when rideing if i have to stop suddenly i use my back brake with my front it cuts my bike out ( Its my back brake that cuts it out , it only happens if i have to slam on , Its only started doing this since i started to lose power ) Also i have been told it could be the rollers thay are on order can anyone pls help me pls i dont no wot else to do