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?

Check the dropout alignment? Or the RD's pulley cage? If both are good, then I'd say it's time for a new chain.

New or used freewheel? Operating in indexed or friction mode?

If it's a used freewheel, it's a worn cog. There is no sense in disassembling any of the Shimano FW's to swap cogs - you're pretty much in the soup for a new replacement.

-Kurt

P.S.: Someone was riding at the bottom of the cluster more then they should have.

One thought - when I stand on my bike, the frame flexes some - could this be throwing it out of alignment? Any scratches on the stay from the chain hitting?

Hmmm. Let's see. The OP has the following setup:

1) 7-speed freewheel
2) 6-speed indexed DT shifters
3) (Deore) XT MTB RD with 105 road shifters

So maybe there's a clue in here?

Try a different seat?

Not if it was working fine before.

-Kurt

OK, another clue. He said the freewheel is new.

new freewheel and using indexing mode. The rd is new also but the chain is not.

It has been mentioned on this forum in the past that a 6 speed index shifter will work on a 7 speed freewheel and it does work fine in the 6 cogs. Obviously, one of the end cogs will not be indexed.

The wear of the chain is not meshing correctly; the chain needs to be replaced as well.

The problem manifests itself on the 13t as the chain has enough grab on the larger cogs.

-Kurt

It could be a stiff link, or it could be that the chain isn't properly centered on the 13 tooth cog (possibly due to derailleur H-L adjustment, or derailleur cable tension).

Just remove that cog.

I would guess it's the "Shram" chain. Sounds like some cheap Chinese knock-off of a SRAM chain (which is pronounced sram, not shram, scram, or S-Ram...)

Thanks for all the suggestions. I suspect the chain as the problem so I expect a replacement is in order.