CVT gearboxes often take flak for the 'Rubber Band' effect. However, I'm not sure what you're describing is the same thing. I understand 'Rubber Band' effect as the delay in acceleration when you suddenly go WOT - The revs wang around to the rev limiter, there's a surprisingly steady increase in acceleration and then a sudden SURRRRRRRGE - It almost feels like the CVT control unit goes "Oh! You want maximum acceleration rather than my usual? Right, err, 'ang on just a sec.... There we go!"
What you're describing is different I think, but I think I've had the same thing too - Usually on part throttle, sometimes on a gentle incline. The rev counter fluctuates a little and it feels not unlike transmission 'shudder', but the oscillation is much slower than that. It almost feels like the auxiliary gearbox is constantly engaging and then disengaging again. Does that sound about right?
__________________________________________
View my fuel log 2014 Mirage 3 CVT 1.2 automatic: 47.9 mpg (US) ... 20.4 km/L ... 4.9 L/100 km ... 57.5 mpg (Imp)