martinfst i don't know how thank you.
Well, the problem seem to be a mail sended in the morning from a user to 50 user with an attachment of 4MB.
Now the top say
top - 14:12:52 up 40 min, 3 users, load average: 27.07, 46.73, 61.66
Tasks: 85 total, 2 running, 83 sleeping, 0 stopped, 0 zombie
Cpu(s): 9.6% us, 1.3% sy, 0.0% ni, 0.0% id, 88.7% wa, 0.3% hi, 0.0% si
Mem: 450892k total, 313392k used, 137500k free, 2864k buffers
Swap: 1317288k total, 265372k used, 1051916k free, 28904k cached
and go down.
Sorry for my english
Thanks again for your support.