Did you try removing some of your ppa's from update manager, sometimes if you have installed ppa's through your terminal this also adds them to your update manager and also affects how software centre reacts when too many ppa's are selected, so Ubuntu Software Centre is not broke but can't install programs due to this. Trust me I reinstalled my OS so many times because of this stupid annoying problem. Eventually I couldn't even use my Terminal.
Try this open dash type update manager, open it,click -settings, then click other software, then uncheck all the ppa's that you've installed and keep your main ones like "Independent (Source code)
I'd uncheck all the ones below source code. I could never could figure out this annoying problem until I solved it with this solution. I really hope this works for you it did for me. Cheers!