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This is known as Periodic Error and is completely normal, not much you can do about it without guiding OR replacing all of the mechanical components within the mount to a higher quality.
It takes about 7.5 minutes for the EQ6 to make one revolution of the worm gear, that means that the periodic error is over a 7.5 minute period. It changes from mount to mount (my EQ6 has a different PE than yours) but they're typically around the 10-30 arcsecond mark.
If your camera has 5 micron pixels and you image with a 80mm F/7.5 your image scale is 1.71"/pixel. If your mount has a PE peak to peak of 20", that is 10 in each direction that is a drift of about 2.5" per minute.
If you can get round stars at 2 minutes then I assume you have about 15" peak to peak periodic error which is reasonably good.
If you have gone over 2 minute exposures, you may just be pushing it too far for unguided.
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