Speed up Ultimate with multiple stores - HELP

We have 5 stores running on our Ultimate (3.06) configuration using a dedicated server from Network Solutions. All is working great, except during the peak hours the server slows to a halt and sometimes crashes.

I have seen lots of posts for optimizing cs-cart for speed. But I haven't seen much information on how multiple stores fits into this equation. We have done the basic stuff (using sqlite, smartoptimizer and configure the config.local file).

But what else should we look at to prevent the server from crashing everyday. Network Solutions keeps blaming cs-cart for being poorly coded, but I am hoping we can help them tweak some server parameters to better optimize their server configuration for a multiple store cs-cart setup.

Help desperately needed,