skipping ultegra 10spd chain

Check the jockey wheels for damage / wear, and that they are in the right positions ( guide at the top, T pulley at the bottom ). Then make sure the chain is the right length, and that any slack is taken care of with a b screw adjustment if necessary.

I didn't want to appear to insult your intelligence by suggesting it was probably the chainring because you had said it wasn't.  In a seperate post and everthing.

If a new chain is skipping you change the cassette, if it continues to skip then new chainring, then considering changing my chains sooner after taking the financial hit.

I cannot say if I have ever seen my chain skipping, it seems to only do it under heavy load, never on the workstand, usually pulling away at traffic lights or going up nasty little hills, more of a very disconcerting shocking feeling.

Though I have managed to change a several single sprockets to keep the rest of a few shockingly expensive cassettes going, knowing that it probably hasn't got long left.  A bit like putting a bit of lube into a freehub and building it back in to give you enough riding time to find a new one, and really kill the old one.

My KMC X1 EPTs seem to be at less than 0% when new using my chain checker.  So little side deflection too, tuff little chains.

But I would be most likely thinking small chainring 

is the mech hanger perfectly straight? Is the rear wheel sat perfectly square in the dropouts? Cassette lock ring really tightly cranked down?

If it doesn’t slip on the same cassette cogs when in the big ring it is almost certainly the little chainring. Check for worn teeth, bent teeth, etc. Can you get it to do it when on a workstand?

PP

Worn drivetrain components can be bafflingly hard to spot, it always seems like the problem is elsewhere!

I think you will find that it is skipping on the small ring and not the cassette. Had the same issu, was convinced it was skipping on the cassette, replaced the inner ring and all fine from then on.

Tricky one. I'd have a good look at the derailleur and jockey wheels and check that the derailleur plate isn't upside down and that the chain is threaded correctly through them. Also, check that the cassette is done up nice and tight (somewhere around 40Nm if memory serves) and doesn't have any movement.

Obvious question, but is it routed through the jockey wheels correctly? If you get it the wrong side of the thingy it can skip a fair bit.