I was testing my raspberry pi with raspbmc and openelec for sometime before deciding to stick with OpenELEC when the 4.0 betas started being released.
Everything worked great but I had some trouble with youtube and in general streaming medias (like Comedy Central shows) as they were casually brining the machine to a grinding halt followed by a manual unplug-plug reboot, or in case of OpenELEC you could wait it to self restart XBMC.
Finally I found a video that was causing it always after e cetrain period of time, and thanks to XBMC debuging logs I noticed that the free ram was just slowly decreasing until the lack of memory totaly annihilated the machine as unfortunately these Pis have only 256MB of ram, half of which is usually dedicated to decode HD videos.
I tried to fix it in various ways. Even adding a a tiny swap partitions on the non-aligned empty disk space left by OpenELEC installer between the system and the data partitions.
Adding them (using parted on the SD card from my laptop) removed completly the problem, but it requires also to add a cron task to add the swap partitions on boot as OpenELEC developers have their reason to avoid id (just search in their forum for the answers).
With some testing I found a compromising setup that makes everything work, and also made the UI incredibly more responsive while playing even it’s set only to moderate overlocking (800Mhz).
I reduced in the config.txt file the gpu_mem to 108 (128-16-4, a value obtained by trial & errors) and by appending to cmdline.txt the parameter zcache. I haven’t played with the device dynamic memory allocation as it’s as time of writing unsopported