cadencesdad

So I had a friend switch out my 53-39 FSA crankset for for a FSA compact 50-34. I have Ultegra 10-sp
rear deraileur and 105 front deraileur. After the switch, My rear deraileur works great in all gears except 7.

When I shift into seven, it tends to want to ride halfway into eight, then slips back into seven. It does this again and again under load.

My friend said to take it to REI, and have them remove a link from the chain. REI mech said it didn't need a link taken out, as that may cause my front deraileur to snap. He adjusted it, and it worked ok in the parking lot. Took it out same day for a mountain ride, and it started doing it almost immediately.

Any ideas as to whats going on?

umd

Cable tension. I'm not sure what your numbers correspond to, so if it is slipping to a smaller cog (harder hear) tighten the cable, otherwise loosen it.

Retro Grouch 

Whenever I encounter a shifting problem that doesn't respond to normal tuneing, the first thing that I do is to check the derailleur hanger alignment. Most of the time that fixes it.

If you have an 9-speed or 10-speed cassette, you need a gauge to check derailleur hanger alignment. Eyeballing isn't accurate enough.

BarracksSi

That's the problem that I had. I got it pretty danged close by screwing a spare wheel into the derailleur hanger and making it parallel with the wheel already on the bike, and the rear shifting improved a LOT.