It seems that there's memory leak on history syncer process.

$ ps aux
zabbix 19771 0.0 14.8 3358516 1196684 ? S Apr24 18:31 zabbix_server: history syncer #1 [synced 0 items in 0.000001 sec, idle 5 sec]
zabbix 19772 0.0 15.0 3373984 1213080 ? S Apr24 18:34 zabbix_server: history syncer #2 [synced 1 items in 0.009665 sec, idle 5 sec]
zabbix 19773 0.0 15.6 3376472 1264740 ? S Apr24 18:42 zabbix_server: history syncer #3 [synced 2 items in 0.004372 sec, idle 5 sec]
zabbix 19774 0.0 15.4 3369596 1243392 ? S Apr24 18:42 zabbix_server: history syncer #4 [synced 559 items in 0.060622 sec, idle 5 sec]
zabbix 19775 0.0 15.6 3378832 1265472 ? S Apr24 18:41 zabbix_server: history syncer #5 [synced 0 items in 0.000001 sec, idle 5 sec]

$ free
total used free shared buffers cached
Mem: 8061620 7865348 196272 0 11976 408812
-/+ buffers/cache: 7444560 617060
Swap: 8388600 5480792 2907808

zabbix_server.conf

VMwareCacheSize=32M
CacheSize=256M
HistoryCacheSize=32M
TrendCacheSize=32M
HistoryTextCacheSize=32M
ValueCacheSize=32M