Server overload after statisctcis calculation

[Ali Raza]

When server calculate the statistics (webalizer + awstats) and run other Plesk builtin schedualed jobs, the server load gets too high and mails stop working at that moment. it is set on 4am to run cron and server gets normal after 6am and still mail services needs to start. I have 750 domains on server. Its 2.0 QUAD CORE 4GB RAM RHLE 64BIT

Please help