You say it's loosening the prop nut right? Is it throwing the prop violently when it happens or are you just noticing that it's loose. The reason I ask is because fourstrokes tend to backfire or predetonate during lean runs. This causes the motor to "kick back" thus throwing the prop violently. I have had it happen more than I care to remember on my saito 72. If this is the case try richening your needles a little.