aerox cutting out

hi i just recently baught a aerox 50cc runs fine booked it in for its mot got to its top speed on the dual carrage way and started to slow down and then cut out anybody know what could be causing this?

 

Sounds like the spark plugs fouled mate.. Does it totally refuse to start now?

 

starts fine its a new spark plug just cuts out when its going and gets upto nearly top speed