I took my bike in to the LBS for an overhaul and supplied them with a new chain/cassette (SRAM replacing like SRAM components) to put on, and they said no problem -- wouldn't charge me any extra to put on components as they take the whole bike apart anyway for an overhaul. (hey, I even saved them from having to clean those components, which they normally would do during an overhaul)
I got the bike back last Thursday and on my first ride Friday I stand to pedal and the chain goes "Ka-chunk!" and I thought that can't be good. Chain was still on, everything seemed fine, I was even in the same gear which is odd because it sounded like it shifted very quickly and very hard. I continued to ride seated, and I shifted through the gears and everything was smooth. Next intersection I stand to pedal and again under load my chain jumps on the rear cassette very quickly. I never saw it, but it seemed like the chain was going up to a larger cog on the cassette and then rapidly jumped back down. This only happened under pressure. It happened one other time when I hit a pretty good bump and was pedaling hard.
I took my bike back to the shop. They put it in a stand and took it through all the gears, which worked very smooth. I explained that it only happened under load (standing) and not during normal pedaling. The mechanic then took my bike for a ride in the parking lot and couldn't get it to happen (he weighs about 50 pounds less than me). I said I'd try and reproduce it and sure enough, was able to get it to jump on cue by standing and pedaling. He looked my bike over and told me I had a defective chain. I said no problem, I have a second chain at home I can put on. CRAP, bought the 991 chain from PBK months ago so wouldn't be able to return it.
I took my bike home, set the new second chain (951) to length, and then put it on. Right away I could get it to jump while standing and pedaling hard, so I took my bike back to the shop, as it obviously was not a chain issue. Then the mechanic tells me it must be a defective cassette. He put on a new Ultegra cassette from their shop and it didn't seem to happen (I took it for a spin too and it didn't happen). So fortunately they had not emptied the trash and they dug my old cassette out and put it back on. The mechanic installed that and rode it in the parking lot and said same result, no jumping. I didn't ride it this time but just took it and left, as I had other things to tend to. That was last Friday.
This morning (Mon) I had time for a short ride and the first time I stood to pedal I had the same jump. So I took my bike back in after my ride. I had read on this forum over the weekend from others with a similar problem the possibility of the cable tension not being set properly. One thing they DID do to my bike during the overhaul was to put a new rear derailer cable on. I suggested this to the person I spoke with (different guy) and he indicated that couldn't be the issue because the cable "wouldn't cause the chain to jump like that." I surely don't know as much as the bike mechanics, but don't understand why the cable couldn't be the issue. So far it is the only other variable in the equation that was changed. But he asked me what I wanted to do and I explained the problem again (which he already knew) and said I wanted my drivetrain to work properly, like it did before I brought it in for the overhaul.
So if anyone has any suggestions on what this might be, I'd like to hear it. I believe the new chain and new derailer have been ruled out since replacement parts exhibit the same behavior. Also, this shop has mixed up brake/shift cables on me on the past. If they put a brake cable in for the shifter, would that possibly cause this problem? I didn't think to check that until now, and my bike is back in the shop at the moment. But I can compare cables and can tell the greater thickness in a brake cable.
It seems to me the cable tension might be the issue. I was surprised the guy I spoke with today seemed to think that couldn't be the issue. He sure made me feel dumb for suggesting that could be the problem.
Edit: just want to add this is a 9–speed, and all components (chain and cassette) are all 9 spd as well.