Since im Using KDE Plasma, i have the Problem the system is Freezing randomly over the when the Desktop is locked

kern.log when the problem begins

Oct  5 02:28:06 druuhl kernel: [73499.905933] nouveau 0000:01:00.0: fifo: SCHED_ERROR 0a [CTXSW_TIMEOUT]
Oct  5 02:28:06 druuhl kernel: [73499.905939] nouveau 0000:01:00.0: fifo: sw engine fault on channel 5, recovering...
Oct  5 02:28:08 druuhl kernel: [73501.905656] nouveau 0000:01:00.0: fifo: runlist 0 update timeout
Oct  5 02:28:10 druuhl kernel: [73504.200746] nouveau 0000:01:00.0: fifo: SCHED_ERROR 0a [CTXSW_TIMEOUT]
Oct  5 02:28:15 druuhl kernel: [73508.495393] nouveau 0000:01:00.0: fifo: SCHED_ERROR 0a [CTXSW_TIMEOUT]
Oct  5 02:28:19 druuhl kernel: [73512.790100] nouveau 0000:01:00.0: fifo: SCHED_ERROR 0a [CTXSW_TIMEOUT]
Oct  5 02:28:23 druuhl kernel: [73517.084810] nouveau 0000:01:00.0: fifo: SCHED_ERROR 0a [CTXSW_TIMEOUT]
Oct  5 02:28:27 druuhl kernel: [73521.379625] nouveau 0000:01:00.0: fifo: SCHED_ERROR 0a [CTXSW_TIMEOUT]

when the Desktop is unlocked over night there is no freeze. Some Discussion about this describe problems with Fullscreen Application like Libreoffice, this would match with my research.

Im Using Neptune Linux - Debian with KDE Plasma and i have the Problem with Kernel 4.4.13 to 4.4.20.

Linux druuhl 4.4.20 #8 SMP PREEMPT Thu Sep 8 10:25:38 CEST 2016 x86_64 GNU/Linux
kdeplasma 5.6 to 5.8
xserver-xorg-video-nouveau 1.0.12
xorg 7.7