gilera dna 125 loses power when high revs?

i need to go on a dual carriage way to get to work but find that when my GILERA DNA 125 gets to top speed (68mph) it'll stay at that spped for a bit then the power cuts out then will come back on at about 45 to 50mph, i was told it may be the rev limiter but i cant get back to that speed before it cuts out again, (doesnt cut out at the same revs every time), any idea what the problem is and how to sort it, its bl**dy dangerous cos d*cks in cars speed past way to close.