A little two-stroke in the petrol shouldn't do much harm. If it is that you should be able to smell it in the exhaust. Sloshing the tank about and mixing it up might reduce the effect, but the first thing I would check would be the spark plugs. Changing them is never a bad thing. I give mine 6000 miles and put new ones in. I sometimes notice them going off at slow speed running towards 6000 miles.
You might see some evidence of oil on the old ones or maybe even a wet plug?
An Italian tune up might be the simple answer at the end of the day.
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