Since yesterday my Apache server that runs on a Debian machine runs very unstable. Sometiems my websites load and sometimes not. I think it has to do with the memory since my Apache log is full of Out of memory (allocated 262144) (tried to allocate 4480 bytes). I also attached a screenshot of the memory graph.

A server restart resolves the problem temporarily. I looked at the processes that are using memory but the biggest one is MySQL with 6.5%.

Where else can look for the problem?

Edit: I did a free -m right after rebooting and one about 2 hours later. I think the trend is visible:

root@xxx:~# free -m
             total       used       free     shared    buffers     cached
Mem:          4016        731       3284          0         80        200
-/+ buffers/cache:        449       3566
Swap:          459          0        459

root@xxx:~# free -m
             total       used       free     shared    buffers     cached
Mem:          4016       2466       1550          0         92        473
-/+ buffers/cache:       1900       2115
Swap:          459          0        459