if you are using php-fpm it needs to tune the config. like the max children, requests etc.
how is the sites traffic and servers hardware?
monthly
rx | tx | total | avg. rate
------------------------+-------------+-------------+---------------
Dec '12 40.58 GiB | 375.32 GiB | 415.90 GiB | 1.30 Mbit/s
Jan '13 20.22 GiB | 245.32 GiB | 265.54 GiB | 1.20 Mbit/s
------------------------+-------------+-------------+---------------
estimated 29.18 GiB | 354.03 GiB | 383.21 GiB |
daily
rx | tx | total | avg. rate
------------------------+-------------+-------------+---------------
yesterday 924.33 MiB | 10.97 GiB | 11.87 GiB | 1.15 Mbit/s
today 342.13 MiB | 3.74 GiB | 4.07 GiB | 822.78 kbit/s
------------------------+-------------+-------------+---------------
estimated 711 MiB | 7.78 GiB | 8.47 GiB |
And server hardware:
Chipset: 2 x Intel Quad Core E5504 Xeon CPU, 2.0Ghz
RAM: 4 x 2Gb DDR3 ECC
HDD: SAS 3.5", 15k rpm, RAID 10