Sorry, I can't be too specific here because I'm no mechanic, I took my bike into the shop when the cable broke. the mechnic there is very experienced, he works with our (New Zealand) Pure Black pro team. he says that design of the Shimano brake/shifter lever is a problem because the inner cable does a sharp bend where it attaches internally (sorry, can't remeber the correct wording he used) to the shifter mechanism. Cables tend to break there, and if it's slightly frayed or not well lubricated it can cause problems with the gear shifts. Hope this helps, sorry for my lack of correct terminology. Good luck with the problem.