The only time I have ever heard of someone having this problem was with an older version of mojoPortal.
Have you upgraded to the latest mojoPortal?
Are you using any custom features? Any custom features you have impemented are potentially the cause, and I have no way of knowing about those features and what they are doing in code.
A long time ago in an older version of mojoPortal, there was a bug where if you deleted a page that had child pages, the child pages would still have the parentid of the deleted page and this could cause a problem if the child page was configured to show breadcrumbs. It would consume the processor trying to resolve the bread crumbs for the missing parent page. This has since been fixed so that now if you delete a page, the child pages get their parentid set back to -1 which means no parent page.
If this is the same problem you are having, then you can fix it by running this sql:
SET ParentGuid = '00000000-0000-0000-0000-000000000000',
ParentID = -1
WHERE ParentID <> -1
AND ParentID NOT IN (SELECT PageID FROM mp_Pages)
If that is not the problem for your installation then I have no idea what the problem is. I run quite a few mojoPortal sites and never have any cpu use problem as you describe.
Hope it helps,