My son's was doing something very similar as you described. I looked all over the internet to find an answer. Finally just split the case and found a very tiny chip out of the 1st gear wheel. I think it took longer to locate the chipped gear than to tear down the bike. The part I replaced was #13 on the oem transmission parts fiche. Part #1P6-E7211-20-00. I just got it back together today and the clicking noise is gone. My guess is that you have a chip somewhere in the transmission. I'll try to post a picture or two to show how small the chip was. The defect actually shows better in the picture than in person. Hope this helps.