i love my SR125

Hello, I have a SR125, she is old (K Reg), she has developed an annoying problem on longer than 5 mile journeys, when i get upto 50mph she suddenly slows down to 40mph then 1/2 mile or so later speeds up again, she starts first time everytime and will amble along quite nicley at 35-40, but part of my journey to work involves using dual carriageway and this sudden dip in speed has caused a few near misses. any ideas most welcome