I have a VSP (centos6.1 64bit) with 4gb ram.

It always runs well, but in recent few days, the server become slowly. open a small css file need 22 seconds(2kb). tested in home/office/phone with (IE,chrome,safari,firefox). see in firebug networking

DNS Lookup   ?4?ms
Connecting   ?21.18?s
Sending  1?ms
Waiting  ?115?ms
Receiving    ?9?ms

The connection cost 21.18 seconds

I have checked all the log file, there have no error. top commond, still have free memory.

top - 00:23:15 up 8 days,  3:57,  1 user,  load average: 3.60, 3.42, 3.83
Tasks: 221 total,   4 running, 217 sleeping,   0 stopped,   0 zombie
Cpu(s): 19.3%us,  3.2%sy,  0.0%ni, 76.1%id,  1.4%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   4194304k total,  3247724k used,   946580k free,        0k buffers
Swap:        0k total,        0k used,        0k free,        0k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
32357 mysql     15   0 3710m 835m 6268 S 34.5 20.4  39:14.40 mysqld
 9780 apache    15   0  442m  59m  12m S 33.2  1.4   0:05.69 httpd
 9842 apache    15   0  403m  26m  10m S 16.9  0.7   0:01.23 httpd
 9847 apache    15   0  412m  45m  22m R 15.3  1.1   0:01.00 httpd
 9834 apache    15   0  426m  46m  11m R 13.0  1.1   0:02.22 httpd
 9891 apache    15   0  407m  43m  19m S  8.0  1.1   0:00.33 httpd
 9845 apache    15   0  414m  51m  24m S  6.0  1.3   0:01.53 httpd
 9827 apache    15   0  402m  28m  11m S  3.3  0.7   0:02.69 httpd
 9768 apache    16   0  414m  51m  24m S  3.0  1.3   0:06.51 httpd
 9889 root      15   0  211m  12m 8160 S  2.7  0.3   0:00.32 php
 9702 apache    15   0  415m  55m  26m S  1.7  1.4   0:10.67 httpd
 9844 apache    15   0  413m  47m  21m S  1.7  1.2   0:01.21 httpd
 9697 apache    15   0  414m  51m  24m S  1.3  1.3   0:11.05 httpd
 9778 apache    15   0  414m  53m  25m S  1.3  1.3   0:05.38 httpd
 9772 apache    15   0  414m  51m  23m R  0.7  1.3   0:05.04 httpd
 9823 apache    15   0  415m  50m  23m S  0.7  1.2   0:03.97 httpd
 9837 apache    15   0  402m  27m  11m S  0.3  0.7   0:01.04 httpd

Then, how to check where is the problem and fixed it? I haven't change and config files in these days. Thanks.