Originally Posted by
Collin2424
I already tried without the cable/housing attached. That was my first guess too.
Will a Sora/Tiagra shift as nicely as my RX100? Well, it doesn't shift now but you know what I mean
-Collin-
Tiagra is very close to 105 and is considered good quality and will give you very good service. Sora is decent but a bit heavier and not as well finished. Either is adequate but go for the Tiagra.
Since you've eliminated the cable and housing as the problem, have you thoroughly cleaned and relubed the derailleur's pivots, springs and pulleys? Crud buildup in the derailleur itself could be your problem. I've had several Shimano rd's last 30,000 miles so they don't wear out quickly.
BTW. "8-speed" chain is the same as 7-speed chain. Most 8-speed chains are marked 7/8-speed.