Yup, what he ^^ said. I used to find the bike wandered with me and g/f on and our combined weight was around 150kg. The max payload of the bike according to the manual is 190kg. I would definitely look at uprating the rear spring or shock, the OE one is dire anyway. Maybe going for some thicker fork oil would help a little up that end without spending all the ���
What tyres are you using? I had Mitas e07s at one point that would slide all over. Most other tyres have been pretty good.
For a new spring I would recommend speaking to MR Holland on 01775 766455 as they can manufacture a spring to suit your weight, costs aren't steep at all. They made me one and from memory it was around �80.