Avast malfunction: almost need to restore my computer

My computer is very unstable since yesterday. Frequently can’t access Internet from browsers, and Chrome remote installation program never succeeds, and downloading a commercial software stored in .rar files from a commercial website is always failed. I’m just ready to restore my whole computer, but I suddenly found when I disable Avast for 10min, everything work flawlessly on-the-fly. I’ve been using Avast free and ask my clients to buy it for years. But this is a horrible experience of my using Avast.

My current Avast free version is 2014.9.0.2013 and definition is 140313-1. I’m in Win7 x64.

do you have more then one AV installed?

have you tried avast repair?
controlpanel > ad/remove programs > avast > uninstall/change > repair option…

any change?