I have a stock 2011 Specialized Epic Carbon Comp 29er XL. I'm 47, 6'4" and 230lbs. I ride 1x-2x on weekends, aggressive (ex-novice motocrosser), and in ok shape with little to no training. Been back in mountain biking for about 9 months (yes, I did the Flume Trail on metal Trek mtn bike in 1985).

I work on my own bikes and am having trouble with a rear X-9 derailleur.

I've adjusted the high and low settings to be correct on repair stand and sound correct under riding.

However, the rear X-9 doesn't always go to a mid-gear when shifted and will skip two gears or I have to shift two gears past the gear I want and then back one to desired gear

Another problem I am having is the rear X-9 emits a slight sound like it is about to shift (very slight mis-adjustment) when I place it in a mid-gear. Is there an adjustment that I am missing to align derailleur to cassette when in a mid-gear? High and low gear adjustment seem to be ok.

It also sounds like it I am stripping the cassette when shifting under uphill load.

When traveling down smooth fast single track and when I hit a deep pot hole at speed, I hear the chain slapping around when the suspension compresses.

I see other bikes run a chain tensioner wheel closer to cranks. Do I need this?

Am I out riding the X-9?