How many products and categories do you have?

When you say 'Navigating admin' does this mean any and all admin functions or only certain aspects of it?

If SSL enabled, have you tried with it disabled?

You've only mentioned the admin being a problem, but what about the store front, is this working at full speed, or does it also show similar signs of sluggishness and/or excessive CPU usage?

How long has the site been running and when did the problem start?

What changed?

Basically, I'm trying to get a look at the 'bigger picture', and also trying to get further clues.

100 modules seems to be a heckofalot, and depending on what they are or what they do there could be some sort of conflict going on - however, if these same modules have been working nicely together prior to this problem this potential conflict probably wouldn't apply.

More info needed.

Cheers
RodG