...speeds down every 15 days!!! HELP!

Hi. I have Mikrotik 3.10, and on every 15 day I have strange problems: incoming speed getting down to about 5-10 Mbits/s (I have about 50 Mbits/s channel) until manual reboot. I’ve noticed the problem on version 3.20 and downgraded to 3.10 but no luck. Than I turned off all unused modules. The other modules left enabled:

advanced-tools
dhcp
ppp
security
system

The configuration of the box:

MB ASUS P5B-MX
CPU intel Celleron 420 1,6 GHz; cache: 512 KB; cores 1; 65 nm;
MEM DDR II Hunix 512Mb
IDE Flash Disk On Module Transcend 256 mb
Power: Hopley 350w
NICs: 2x  INTEL 100mbit/s PRO.

Graphs of the last fault attached
Incoming speed
graph_image.php.png
Processor usage
graph_image.phpproc.png
Memory usage
graph_imagemem.php.png
LOGS ARE CLEAR - no errors

Any idea? Really need help!!!
I’ll post any configs if needed!! Thanks!!!