apache为何频繁宕掉？

最近apache（2.0.55) 总是自己宕掉，手工重启apache后恢复，但几个小时后又会宕掉
摘录一段error.log为：
[Wed Sep 10 09:36:39 2008] [warn] (OS 121)信号灯超时时间已到。  : winnt_accept: Asynchronous AcceptEx failed.
[Wed Sep 10 09:36:39 2008] [warn] (OS 64)指定的网络名不再可用。  : winnt_accept: Asynchronous AcceptEx failed.
[Wed Sep 10 09:36:39 2008] [warn] (OS 64)指定的网络名不再可用。  : winnt_accept: Asynchronous AcceptEx failed.
[Wed Sep 10 09:36:39 2008] [warn] (OS 64)指定的网络名不再可用。  : winnt_accept: Asynchronous AcceptEx failed.
[Wed Sep 10 09:36:39 2008] [warn] (OS 64)指定的网络名不再可用。  : winnt_accept: Asynchronous AcceptEx failed.
[Wed Sep 10 09:36:40 2008] [warn] (OS 64)指定的网络名不再可用。  : winnt_accept: Asynchronous AcceptEx failed.
[Wed Sep 10 10:34:51 2008] [warn] (OS 64)指定的网络名不再可用。  : winnt_accept: Asynchronous AcceptEx failed.
[Wed Sep 10 10:37:51 2008] [warn] (OS 64)指定的网络名不再可用。  : winnt_accept: Asynchronous AcceptEx failed.
[Wed Sep 10 10:40:19 2008] [warn] (OS 64)指定的网络名不再可用。  : winnt_accept: Asynchronous AcceptEx failed.
[Wed Sep 10 10:57:22 2008] [warn] (OS 64)指定的网络名不再可用。  : winnt_accept: Asynchronous AcceptEx failed.
[Wed Sep 10 10:58:56 2008] [warn] (OS 64)指定的网络名不再可用。  : winnt_accept: Asynchronous AcceptEx failed.
FATAL:  erealloc():  Unable to allocate 503863 bytes
[Wed Sep 10 11:30:06 2008] [notice] Parent: child process exited with status 1 -- Restarting.


原来怀疑是访问量大，线程不够，已经修改了ThreadsPerChild  改为 800 （原来为25），没有用。
有谁碰到过这个问题吗？谢谢拉