I hope I'm explaining this right so it makes sense. I got the Cannnondale Synapse 7 with the Crankset Shimano Sora, 50/39/30 and a rear cog of SRAM PG-950, 12-26, 9-speed, It seems when I'm on the 39, middle gear, on the crank I get a kind of slippage/skip on the chain when I'm pedaling. The chain doesn't fall off, but its almost like its missing teeth and having to re-catch the gears. If I'm on the big wheel of the front crank its perfectly fine, no slippage. I don't do much riding on the small one, but from what I've tested it works fine.
When I first got the bike it was riding fine after about 100-120 miles it started doing this action, I brought it in to the bike shop I bought it at and they did a full tune up on it and the problem went away. Now I've gone another 150ish miles and its starting to do it again.
Is this normal? I don't ride really hard, but I do ride often. I'm starting to do longer and longer rides so having this issue pop up every 120-150 miles could be annoying, unless its common and I need to learn how to adjust it.
Sorry for the long winded post, hope it made sense.
Thanks in advance.