So I just bought a new Felt F65 with all ultegra shifting and an 11-23 rear cog. Everything worked perfect and shifting was very smooth until I cleaned the bike and the drive train. That's when all of this started happening. The chain would not set on a gear and it just shifts up and down by itself. I took it back to the shop, they tuned it up, and the problem disappeared for about 40-50 miles of riding...then surprise... its back, but in a different form. For some reason, the chain will not set on the 20 and 22 tooth rear gears (I may be slightly off on those numbers...but its the 2nd and 3rd gear from the 23 tooth gear). It's even worse if I have the chain in the large chain ring in the front, or going up any hill. The cog looks ok and not bent, and the rear derailer will put the chain where I want it to... it just won't stay on those two gears. Anybody know why this crap is happening? Any help would be greatly appreciated. (sorry for the long paragraph)