Magento Speed Server Optimization Expert

We are using Magento 1.4.1.1. Optimized mysql with query cache, innodb buffer size etc. Also, using Gzip content and APC as cache. session stored in file system. also did some block caching. images are cached with far future expired date, combined js and minified css already. also, follow standard seo / xhtml coding. However, the server was down a few times in this month. (not much traffic increase, also sometimes downtime is in non peak traffic period). we also checked log and don't see malfunction / access. Need serious Magento and Server expert to help fix the problem. Only apply if you are expert in this area. We are looking for expert with knowledge in server, speed optimization and Magento.