ghost server?

hi. i menage quite popular internet wortal (http://hip-hop.pl) and I use mysql and php 4.0 as the main generator of all webpages. since few months every day i have some problems with the server (AMD Duron 700MHz, 256 Mb RAM, ABIT KT7-RAID, Intel Ether Express 100+). operating system is FreeBSD 4.3 the internet provider is checked, fast and good. i know this because on this internet connection is linked another server (rather low traffic) and it works non-stop. this problem grow every day. the problem is that this server disapears for few minutes, and then it apears again. it happens only in hours of main usage (i know it from hourly satistics). when it apears it works rather fast. the problem is growing every month because our statistics go up. when the server stops none of the installed services works. and what is amazing there is no errors nor warnings in the logs. i was wondering if its is because of mysql or other service installed on the server. has anyone the same problem in the past? what is the reason? please help.