Sigma lens operation

Can anyone please help?

I have a Sony 5n and a Sigma 30mm F2.8 lens. In manual mode the lens is very notchy and does not turn smoothly. It focuses nicely but turning the focus ring is not smooth. There is no grating sound - just parts of the turn are stiff.

Is this a fault or a feature? Auto focus is no problem. Apart from manual which I use frequently the lens seems fine. Should I return it?

Thanks in advance for any advice.