Drupal Caching Issue

Hi Drupal support team,

I'm managing a Drupal web site, which is using Pressflow Drupal 6. The site is having good traffic. The site architecture as follows

1 load balance and 2 web servers and 2 database servers. The Mysql databases are replicated.

The issue is - the website go down 1 or two times in a day and then I need to log as admin and then clear the Cache. Once i clear the cache the site become available to public.
Following are the configurations made on the Cache interface

1) Caching Mode:
Normal(recommended for production site, no side effect)

2) Minimum Cache lifetime:
10 Min

3) Page cache maximum age:

4) Page Compression:

5) Optimize CSS files:

6) Optimized Javascript files:

Could you please let me know what is causing issue, Please email me your eply as I dont have any account in this site

kuriakosekk @ gmail.com