where is memory gone (no, not buffers or cache)
If you're oom-ing, you almost certainly have an application that has a memory leak. Often the offender is the one the kernel selects to kill (but sometimes not).
Have you tried something like memtop?
you can execute
find /proc -name oom_score -exec echo -n "{} " \; -exec cat {} \; | awk '{print $2, $1| "sort -n" }
and check which app is candidate for oom kill -usually it consumes more memory- It seems to me like an app running wild. Either allocates too many descriptors or some threads are not ending properly.