High cpu spikes

Hi guys,
I've investigated high cpu usage (3820 with ACME Net-Net 3820 Firmware SCX6.4.0 MR-3 Patch 1 (Build 320)) for few days and don't exactly know, why I have spikes up to >90%.
I don't use any HDR and the sbc works in dialog-transparency mode. Log-level is also poor - mostly MAJOR. The only explanation for me is that, I've got too much signalling message per second (anyway I hope it is not true, becouse it is quite small amount in our network). The problem could be, that I have 3 SA (peering based on realms), which can send even 100 INVITE/s - so I create constraint to 24 (max-burst-rate). How I can find the reason of this cpu behavior  ? How many CPS should SBC handle in this scenario ?