Join Date: Feb 2005
Bikes: Trek 5500, Colnago C-50
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This is often caused by 1) improper cable tension, try adjusting the tension with the barrel adjuster found at the RD.
2) dirty gunked-up shifters, try hosing out the shifters with WD40
3) dirty and worn shift cables and cable housings, especially the short housing just before the rear derailleur.
You may need new shift cables and housings, always replace the housings if replacing the cables.
BTW, a downshift is going to a larger cog on a cassette or freewheel.