[CentOS] High load since passing from rhas3 to centos4.8

We are migrating HP servers from RHAS3 to centos 4.8. Since, the load average is reaching really high values. Under usage we see loads of 15-20 instead of the 0.5-1.5 we were used to.
I searched a lot for information on such an issue. Iostat, vmstat, top, ps. But I don't get significant hint.
Cpu usage remains low, IO wait remains low, no disks are lagging, no swap usage.

Are there some tool / other way to diagnose why the load average is high? Like which processes are waiting, where they are stuck. Are there calls to drivers or system process that are slow?

Thanks
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