Scooter with CVT transmission has intermittent jerkiness at low speed

I have a 2016 Sym Symphony ST 125cc automatic scooter which I bought second-hand from eBay a few months back. Having never previously owned a scooter I decided to get it serviced by a Sym approved garage before using it. Since then it’s mostly been OK except that on some days I have trouble with it being very jerky at low speeds when using light throttle input and this can make it very difficult to control. I’m guessing that either the clutch or the belt is sometimes slipping but I’m not sure.

I took it back to the the garage and they looked at it again and could find nothing wrong. But on that particular day it was acting fine so I couldn’t demonstrate the issue to them. They did say they would give the transmission a quick clean though

Maybe it was a coincidence but the next week it seemed to be better than ever with no jerkiness at all. I thought that maybe the problem was cured but today after not having used it for a few days due to rain it was just as bad as before.

As a side note, I have also found that sometimes the bike is hard to start and needs several attempts yet other times it starts instantly. I always start in on the centre stand and have not noticed the back wheel spinning at idle revs or anything like that though.

Is there anything I can easily check to determine the cause of the problem?