My gixxer does this from time to time. Shows 2 when in 1, 3 when in 2. Had the sensor replaced but it started happening again after a while. On inspection I found that when crud gets built up around the front sprocket it gets flakey. So, when it starts happening I simply pull the sprocket cover and clean the goop and grime away. Presto, accurate gear indicator again. Not really sure if this will help your situation but thought I'd pass it along.