when it does it, turn the engine off and pull the plug straight away, check what colour it is
you could get an Outer Rotor kit for it,
http://www.dhz.com.au/buy/racing-out-rotor-kit-adjust-timing-semi-digi-cdi-harness/ENP-27.LF150
it will rev a bit quicker, and they give less kickback when starting, better for your kickstart gears etc
do you have a multimeter ?
you can measure the resistance in your ignition coil to check it's ok.
set it to 200 Ohms, put one lead one each terminal, and it should read about 1 Ohm
then set it to 20000 Ohms, put one lead to the postive terminal, and the other goes into the coil lead (with the spark plug cap unscrewed from the end of it)
you should get about 3-5 Ohms there
if either reading is a lot higher, then the coil is on it's way out.