I have tried this now:
- Removing the SD card
- Removing the SIM
- Used another battery without NFC
- Temporarily used a CPU tuner app and switched the CPU temporarily to highest level.
But the problem is still present.
Additionally I found another new problem beside of the stuttering:
- Two heavily used apps doesnt react anymore and needed to be started again (Threema Messenger and Viber messenger). Seems like the system was under high load / low RAM and stopped the apps. But RAM consumption of the device is moderate.
- In YouTube sometimes videos are shown in a damaged way and just in black/white. After restarting the video it is shown correctly.
Im thinking of two last chances:
- Is there a way to create log files of the system and is there somebody who is able to find the problem seeing these logs?
- Or the last way: Switching to another ROM. In this case I can not use the backups created by Titanium, right?
Of course you can restore user apps and related data from one ROM to another. And for instance contacts, SMS and call logs can be saved and restored with MyPhoneExplorer. Restoring data of system apps may work but may cause issues.