Saving Old Seagulls

Hi seagullers. i have a problem with my silver centry 75 (electronic ignishion) the old gull runs for about half an hour at about half to 3/4 throttle for about 30 min then just cuts out dead. she starts straight away again but the cuts out every 2 mins or so . if you leave her to go cold she runs for 30 mins again . any ideas, was pos thinking electrical breakdown some where . any ideas on how to test ignishion modual . ( she is running on new plug and 25-1 mix. thanks gadget