G'day Cheese,
Questions off the top of my head that might relate:
1. What fuel are you using?
2. How long have you had the fuel?
3. What 2-stroke mix ratio are you using?
4. Does it happen only while mowing or does it happen even if the blades aren't touching grass?
5. Have you checked the vacuum hose and carby for cracks/leaks?
6. Has the carby been serviced and had washers added to it?