xtz 750 runs great for 10 mins ?

hi all

just spent some hard earned money on a 1990 xtz 750 and all seemed well initially (great fun bike )

starts fine but after 10 mins or so of riding perfectly on the 3 occasions i have riddden it , closed throttle going into corner and serious misfire has started and continued all the way home .

bike ticks over ok but blipping throttle on and off eventually just coughs and dies .

any advice will be greatly appreciated

filters are new , have plugs to fit tomorrow.

regards martin