sti gear cable

am in the process of upgrading my groupset. the current cables seem to need regular adjusting. is this because theyre very probably cheap and nasty? if i spend a few quid (dura ace cables 20 quid) is it worth it for performance and durability. any comments appreciated, thanks.
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Whatever state the cables are in, the limit screw on the rear mech should be set so it can't go into the rear wheel.
Might be cheaper in the long run to save up then look out for a discounted groupset which should come with all the neccessary cables.
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how would i know if it is them?
Also, sometimes there are multiple barrel adjusters in the system. I have found that it all works a bit better if you wind all but one adjuster all the way in, so you are making your tweaks with just one screw.
If you can get pics of your shifters and derailleurs then it might help.
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will try the marking suggestion-thanks again
Check the limit screws on your rear mech, they should stop this happening regardless of the cable tension.
From: http://bicycletutor.com/adjust-rear-derailleur/
Low Gear Limit Stop
The L-Limit screw prevents the guide pulley from shifting any further past the lowest gear and into the wheel spokes. Shift down to the lowest gear, step behind the bike, and check how the chain rides on the sprocket. If it looks like it wants to ride into the spokes, tighten the L-screw clockwise until it is centered on the sprocket. If it looks like it wants to shift down, loosen the screw until it lines up. As an extra precaution you can use your thumb to gently push the derailleur body and make sure the chain will not run into the spokes, as this could obviously have a nasty effect on both you and your bike.
Has your bike been dropped or crashed? Shifting problems caused by a bent derailleur hanger are quite common. The result is poor shifting even after all other adjustments have been made.