This started a couple of weeks ago and has gotten worse. I'll be using my phone normally and it will just freeze. No warnings, no messages, nothing. If I wait around 5 minutes I'll be booted back to the lock screen and the message "Unfortunately the process com.android.systemui has stopped." The phone will then function normally. Now when it freezes I just hold down the power button until the phone restarts since it's quicker.
I haven't installed any new apps recently so I don't think it's down to that. Sometimes before the phone freezes it will stutter badly for about 20 seconds. Would appreciate any help. Running stock 4.4.4.