Just noticed that the bike has been stood for a couple of years.
There is a big possibility, even more so if it's been stored outside, that there could be seized chain links.
Normally felt when riding as a knocking feeling through the left foot as the bad links try and go round the front sprocket at an awkward angle.
Again this is something that can be spotted visually. With the bike on the centre stand just look at the lower part of the chain and all the links should be equally horizontal. A sticking or seized chain link tends to stay put in a slightly diagonal position, and will basically just look wrong in comparison.
Once this happens the damage is done and it'll never really be normal again even if you persevere and try and free up the link with a bit of elbow grease.
A chain that is neglected, even from new, doesn't take long to go wrong if the conditions are right, like excessive rain, salty sea air, etc.
If this is the case then i would get a new D.I.D. X Ring chain & sprocket kit from a reputable company, also making sure you have the new 12mm front sprocket nut to fit as well, rather than the iffy 9mm one.