Try these steps
1) Hard reset(keys-vol up, down, power, vol down)
2)Uninstall apps (apps shown in extras section in settings like extras+info , equalizer (is it working in 550idk)by using storage app
3)Change app download path to "phone" instead of sd card by storage app.
4)Change battery saver "ON" settings to 100% (saves battery as well as to avoid crashing and freezing pbs for smooth operation).
After trying this i am now able to use my nokia 520 smoothly and download apps also smoothly as you stated in the question!!!