I'm having exactly the same problem! Netflix is fine on TiVo, YouTube is fine on the iPad. But lagging every 20 seconds is infuriating. Hopefully someone knows how to fix it, or I might have to get a different set top box.
I note your comments but looking through the Virgin Media Community, I have found this problem of stuttering YouTube reported a number of times as far back to July 2015 with a reply from Virgin Media the problem has been identified. It is now April 2017 and still the problem exists. Perhaps if customers of Virgin Media demanded a rebate on monthly fees due to not receiving the complete package, something may be done.
Could someone at Virgin Media please get their act together and solve the problem.