Phantom shifting Specialized EPic
 

Bought second hand, here is what I have done so far :
Tightened hanger and checked with PArk Tool Derailleur alignment, new gear cable,adjusted B screw. Running 34 teeth on back, XTR rear mech.
About to check wheel dishing/
Is it normal to have rear gears skip slightly on the full sus EPIC.
Any ideas greatly appreciated.

Is the chain just skipping on the chosen cog under load or is the bike really shifting from one cog to another? Does it happen with all cogs or just specific ones?

Skipping under load on top gears. Have fitted new chain and cogs. Perhaps the xtr mech cannot handle the 34 cogs on back. B tension screw fully in but derailleur very close to COG.
Thanks

Check to make sure all of the shift housings are in good shape and are long enough. A section of housing that is too short or dirty/kinked can cause the bike to shift when the suspension cycles.

That's what I was going to say too. Also, try to see what happens when you fully compress the rear suspension.

+2. If the problem persists, you might even try installing a little device called an Avid Rollamajig (I guess they're still available). It goes in the last loop of cable just before the rear derailleur, notorious for helping full suspension bikes with ghost shifting problems.

+1 on the Rollamajig.
Oh yeah and thoroughly check out that crazy f/der mount too.
Lots of places it could come loose.

With an epic, you have to change the housing on the rear mech because of the bottom bracket routing. I wish Specialized would quit routing the cables that way... to many issues in the mud.

I don't know if you ever figured this out, but I was just searching for answers to the same issue. I recently built up an 05 Epic Comp frame from bottom-up. I love everything about the bike, but when the rear suspension is active, I'm getting ghost shifting- almost always it's trying to move the r/der up (easier gear), so it seams that it's pulling tension on the cable when the susp is active. I've tried a number of different things to get rid of it, but haven't found a solution yet and it's driving me crazy- mostly I feel it on fast bumpy sections, and on bumpy climbs (basically any time I'm pedaling through bumps). I know this has to be a resolveable problem, because I did a ton of research on this bike before I bought it and nobody even mentioned this issue, so it has to be with my build/ configuration. I'm running an XT rear der and xt shifters, nothing out of the ordinary. I'll try to re-route with longer housing both at the bottom bracket and at the rear der...