Although my rear derailleur is properly indexed it's suddenly started to double or even triple shift on an upshift - i.e. when moving to harder, smaller cogs on the cassette.
This happens whether I'm riding or even if the bike is completely stationary and I'm only moving the shifter in isolation - I can feel and see the display on the hood jump 2-3 positions. It only happens in what would be the mid positions of the cassette - the very bottom gear and the top few gears aren't affected when I shift up from them.
Downshifts to larger, easier cogs are completely unaffected.
I've tried peeling back the brake hood and giving the mechanism a good blast of penetrating lubricant but to no avail.
Is this likely to be a shifter only problem and what if anything can I do to rectify it?