slow down and surge forward at constant throttle

Hi all,
It has happened to me a few times over the last weeks, all in 4th gear and at constant throttle that the bike has slowed as if releasing the throttle grip completely then revved up again (to higher than it was before) and then carried on fine…

So far it has not caused any problems but I’d rather like to know what’s going on so it can be sorted because if i was leant over when this happened it could unsettle the bike.

I will look to lubricate my cables over the weekend as this is something i have neglected to do so far…

Does anyone have any ideas as to what’s wrong?