My 2000 Cali bassa has 22k miles and since the weekend I've had a downshift problem when the bike warms up. Gearbox oil present and accounted for. Gearlever is sluggish to return back up to normal position after a down shift and sometimes needs a nudge from my boot. Checked linkages and lubricated in case some winter gunk and corrosion is causing it, no difference. Could this be the return spring not behaving as it should? Upshifting becomes clunky around the same time the downshift problems arise, especially between 1st and 2nd on the upshift.