Hello Faizan,
you should provide more details to get better help.
post a filtered version of the C1841: remove username and passwords and mask public ip addresses.
If you reach the router via an IPSEc SA you can face the SA expiration (in terms of time or traffic volume) and then when you ping it you trigger the creation of a new IPSec SA.
Or it can be just a problem of ARP entry timeout when using only static routes.
this is just an example, without knowing the configuration is difficult to say something.
Hope to help
Giuseppe
