Hi falko,Thanks fot your interest.
Non, I don't think it's a memory problem:
total used free shared buffers cached
Mem: 66084500 4684564 61399936 0 305304 3084316
-/+ buffers/cache: 1294944 64789556
Swap: 1046520 0 1046520
And yes it's a physical machine not a VM one.
I tried to have a look at all my logs, but I don't found anything...