The drivetrain is Triple front, 7 cog freewheel 13/32 mega gear, Shimano XT normal low rd, Shimano 105 6 speed indexed dt shifters, Shram chain.

The chain skips on the 13 cog only and on both large chain rings while under any load. It seems to work fine on the repair stand with no pressure. I have moved the high limit screw in and out and it has not fixed the problem. The chain is not worn and the freewheel is new. The frame has verticle dropouts so I can't try changing the rear axle location.

Does anyone have any suggestions for a fix?