check server issue, possible security or DoS attack issue on linux server

check on my linux server.. for past week it seems to slow down and get off line every night, specially between 10 p.m and 11:30 p.m CST time. possible DoS attacks from a user since its a chat site. need to check whats causing the issue and if its an attack or not, and if not need to know exactly why is this happening. its happening now as i type this again, it becomes very slow and users cant get in, and it give error establishing database connection. I need to do it now in the next few minutes, so if you are not available now dont bid please. if you dont know what you doing dont bid please. i will only pay after the problem is solved and you find out what causing [login to view URL] is a small project, i need to do it now.