Server Overloading

I have a client that is running WordPress 2.9.1, they are a fairly popular site averaging around 1,000 visitors a day. Since adding them to my VPS about a week and half ago we have been running into major server overloading issues. My server tech guys and I have been trying many different suggestions they have found here (optimizing MySQL and PHP, adding eAccelerator, trying different themes, turning off widgets, etc.) but we are still seeing server spikes.

After some googling and searching here, I found a lot of people have had issues with wp-cron causing spikes, but most information I have found is outdated or never answered. After I forwarded this information to my tech support they started monitoring and recording when the spikes were happening. What we found is almost every hour, on the hour, and sometimes at 10 after the hour, the server load is spiking very high.

My question is this wp-cron that is causing this problem? Has anyone else had this type of issue and successfully resolved it?

I do have other clients on the same VPS that are running various versions of WordPress but they just dont have as much traffic as this new client.

Any info or suggestions from anyone would be greatly appricated. Thanks in advance.