hmm..I own a newer model Navigator, but I'm not a fan of it at all. They suck! Could be more than a few things including proper re-install of the parts. But, the main things are 1) proper suction (clear restrictions), 2)sand in the a-frame bearings (new bearings are funny) 3) gearbox (if you have opened it up, check the primary (small) bearing; also there is small plastic piece(can't remember if its a drive/secondary bearing) in the bearing housing; proper placement is tricky (find a diagram of your model? and look into that) and avoid opening the gearbox in the future. Good luck