Originally Posted by rccardr

Sounds like you're getting some cable rub or binding as the cable tension increases. Replace the cable and housing if it's not new.
If it is, check the length of the loop between the RD and cable stop. Could be too long or gummed up. Derailleur housing is always better than brake housing or a coiled spring thingy back there, too. I'd also check the RD pivots, clean well and make sire there isn't some fine dirt, sand, etc. gumming up the works at full extension.

Those shifters are pretty darn near nukeproof and should give you a solid single click for each gear shift.