from the day that i disabled EVENTS mod, it take effect on the problem
and i see daily max two cron.php running together.
the problem reduced but not resolved completely.
I also disabled RSS feeder to check again
thanks for helps.
i will paste result here again
not upgraded to 3.8.9 yet
--------------- Added [DATE]1427145831[/DATE] at [TIME]1427145831[/TIME] ---------------
i upgraded my vb to ver 3.8.9 and it seems problem solved.
thanks to all for help