how to solve cache problem ?

hello Friends,

This is cache date Record for my one website.
Before I found this Problem, my site cache on every four - five Days.

this is one example of my site:

17-Aug-12 : 11 Aug 2012 03:15:09 GMT
18-Aug-12 : 11 Aug 2012 03:15:09 GMT
20-Aug-12 : 14 Aug 2012 19:52:33 GMT
21-Aug-12 : 14 Aug 2012 19:52:33 GMT
22-Aug-12 : 14 Aug 2012 19:52:33 GMT
23-Aug-12 : 21 Aug 2012 14:07:11 GMT
24-Aug-12 : 21 Aug 2012 14:07:11 GMT
25-Aug-12 : 21 Aug 2012 14:07:11 GMT
27-Aug-12 : 21 Aug 2012 14:07:11 GMT
28-Aug-12 : 24 Aug 2012 02:41:01 GMT
29-Aug-12 : 24 Aug 2012 02:41:01 GMT
30-Aug-12 : 24 Aug 2012 02:41:01 GMT
31-Aug-12 : 24 Aug 2012 02:41:01 GMT
1-Sep-12 : 24 Aug 2012 02:41:01 GMT
3-Sep-12 : 24 Aug 2012 02:41:01 GMT
4-Sep-12 : 24 Aug 2012 02:41:01 GMT
5-Sep-12 : 24 Aug 2012 02:41:01 GMT
6-Sep-12 : 24 Aug 2012 02:41:01 GMT
7-Sep-12 : 24 Aug 2012 02:41:01 GMT

But I found same problem in my more then 10 sites.

why suddenly this happen to my sites?
what are reasons behind this?

what is solution for this problem?
how can my site again cache on every four - five Days.