VPS Crash! 90% memory is using

[matthew.h]

I have 2 GB Memory on my VPS
Websites that i have hosted are not using much memory. everything was fine but since 5 days ago my vps just crashes and i cannot access it with ssh or anything. it just returns timed out.
before it happens memory is always on 90% or 80%
the only way is to hard reset from client panel!
once it comes up memory is around 30, 40 and everything is fine for some hours

I am using nginxcp and I have CSF installed!

Any one any idea why is it like this?
these websites do not use high resources! i think something eats my memory to die!

here is my top output

top - 13:38:10 up 17 min,  2 users,  load average: 0.03, 0.14, 0.15
Tasks: 118 total,   1 running, 117 sleeping,   0 stopped,   0 zombie
Cpu(s):  6.9%us,  2.0%sy,  1.3%ni, 78.7%id, 11.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   2054988k total,  1340132k used,   714856k free,    61288k buffers
Swap:  2097144k total,        0k used,  2097144k free,   449060k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
    1 root      20   0 19328 1528 1220 S  0.0  0.1   0:00.51 init
    2 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kthreadd
    3 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 migration/0
    4 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ksoftirqd/0
    5 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 migration/0
    6 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 watchdog/0
    7 root      20   0     0    0    0 S  0.0  0.0   0:00.00 events/0
    8 root      20   0     0    0    0 S  0.0  0.0   0:00.00 cpuset
    9 root      20   0     0    0    0 S  0.0  0.0   0:00.00 khelper
   10 root      20   0     0    0    0 S  0.0  0.0   0:00.00 netns
   11 root      20   0     0    0    0 S  0.0  0.0   0:00.00 async/mgr
   12 root      20   0     0    0    0 S  0.0  0.0   0:00.00 pm
   13 root      20   0     0    0    0 S  0.0  0.0   0:00.00 sync_supers
   14 root      20   0     0    0    0 S  0.0  0.0   0:00.00 bdi-default
   15 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kintegrityd/0
   16 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kblockd/0
   17 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kacpid
root@xxx [~]#

please help me

[PlotHost]

Run the top command when the memory is 80-90%.