How many users are using the Shout Box at the same time?
I just enabled it again and UP, server load up to 90% again :-(
Maybe it's just to popular at my site.
If it's a proven cause of the spikes (which you've ultimately proven it is) and your users enjoy the option of having a chat for your forums, you could use one of the (soon to be 3) alternatives available.
1. My pJIRC addon at UBBDev, it's free all around, and uses an IRC Server as a backend.
2. The Raidersoft addon at UBBDev, the raidersoft system is not free, but the integration information is provided none the less.
3. FlashChat may be a viable option as well soon, I've talked with the author and as soon as he can get ahold of a *legal* copy of UBB.Threads he's stated he'll work on the integration (please note that providing your software to 3rd parties *is* a license violation)