I've got a 1980 Ford yt16 with original deck that is randomly missing stips of grass as I mow. I can't determine if its a result of thicker grass, mower speed, etc. Seems to happen whether I mow in 2 or 3 gear and be independent of grass height, mower deck height, etc. The deck is a 42" and consists of 3 blades. All spin forward freely. Seems like there is something that stops the one or two of the blades from spinning which is causing the patchy results. Also, I can spin the blades backward freely by hand. Any suggestions, could it be a tensioner or the bearing assembly? I have sharp blades and a new drive belt. thanks.