vespa pk 135 please HELP any suggestion

hey my brother has a vespa 135 it is a old style scooter looks quite cool but wont run every time it starts its revs will jump all over but ones it gets upto a certian revs it will go burrrrr ( dnt no how to explain it) and go back to lower revs asif it is getting no fuel but it is because every time i take spark plug out it is soaked could this be low compression or worn rings? its asif it stops sparking past a certian rev or losses commpression thanks

 

could be ignition related...duff plug, failing cdi or stator, loose wire somewhere or a dodgy connection...