Do the MD5 verification from the sdcard itself, since corruption can happen even while just copying to your phone (actually it's every bit as possible).
Another thing, the OV is fickle about SDcards. You may want to backup the re-format your sdcard from the phone itself (then put everything back on). Always format from the device you intend to use it in, not the computer for maximum stability.
Also, you may want to see what apps are running in the background, one of them may be causing the instability. You may find you have to wipe the app data, or even completely re-install from the Play Store with no data restored to get it working. Also, where you place said apps can be an issue. Some while claiming app2sd compatible, are not and even sd-ext can create issues. Anything running at all times should be on the phone itself, this includes launchers, background apps, and widgets.