I converted my Salsa Fargo from a drop bar bike to a Jones Bar handlebar. I put on a Devore SLX 11speed shifter as the bike is running a 11 speed 11-42 cassette. On the bike stand everything shifts great. In the hills, under load, it pops out of gear jumping up or down and I find it really annoying.

Any suggestions on how to stop the gear jumping? The only things I come up with are to change the shifter (hoping the first was defective) or go to a thumb shifter.

Thanks

Art's Cyclery Blog » Science Behind the Magic | Drivetrain Compatibility

What shifter and derailluer are you currently using? An 11speed SLX shifter should be matched to a 11-speed SLX/XT/XTR derailleur. Using any other derailleur is likely not going to work.

Using an SLX 11speed shifter and an XTR derailleur for 11 speed.

Could it be a cable tension issue as this is a new build?

Shifting well on the stand but popping under load sounds like a worn out cassette/chain/chainring issue.

It is a new build with new parts. I did notice that there is no barrel adjustment on the derailleur, but one on shifter. Do I adjust that?

The barrel adjuster on the shifter will do the same thing as one on a derailleur, if your cable needs adjusting to line the derailleur up with the cassette and shift better, try adjusting it.