IFS 9 503 - Service not Available

Hello, 

I have an environment IFS 9, in this as the user load increases - more than 40 users. users start getting error - 503 Service Temporarily Unavailabile while

1. Logging in the application, in few atempts they are able to login
2. While doing any processing they get the same error

Database size is only 6 GB and performance looks fine

Can any one suggest any solution for the same

Regards

Pankaj