Random reboots - sometimes the phone reboots multiple times simultaneously.
Wi-Fi - Speeds drop down to below 1M/bps while an iPhone on the same network has 15 as expected. Rebooting solves issues but it is a pain. On top of that it can be stuck on 'obtaining IP address' for quite some time before eventually connecting.
Would it be the fact that the latest Cyanogeon OS is unstable (YOG4PAS3JL)?