Is there anything specific that executes at this specific time on Monday, perhaps an anti-virus definition update or some other scheduled task?
I would just setup some virtual machine CPU alarms that trigger an email alert when metrics go above your defined thresholds. If you get alerted from all VM's then you know it's not a specific one that's causing it. Depending on the time it happens, if it's not too unsociable, then simply open your performance charts and look to see what's happening at the time. What do the stats look like leading upto the time that this happens? what changed?
As suggested above, ESXTOP can also help you out, but you may find the performance charts easier on the eye.
Out of interest, what time does this occur? Is there anything significant about this time?
Cheers,
Jon