First time posting in this forum; I usually hang out in the road biking (41) forum.
I've been riding a Cannondale Synapse 5 for about three years. It has about 10,000 miles on it and is equipped with 105s from front to rear with a triple chainring on the front. I've changed the chain before, adjusted it properly, and it always shifted as smooth as butter afterward.
The last time I changed the chain (same brand as before), I followed the same procedure as always to adjust the Shimano RD. The only difference this time is that the chain will sometimes jump from the middle gear on the front down to the smaller gear. Typically, it will only do this when I'm shifting the rear from a small gear to a larger one, though sometimes it will happen when shifting in the opposite direction. The chain only jumps gears on the front, never on the rear, and only does this when shifting. Once it has settled into a gear, it stays there no matter what the load. I have noticed that when shifting the rear from a smaller gear to a larger one, the chain seems to flex more than it used to and I'm sure that's related to the problem.
I'm hoping someone here has some ideas on why this is happening. Is it possible that the last time I changed the chain, I could have miscounted the number of links and may have one more link in the chain than is optimal? Could that be causing the problem? Any other ideas?