Thank you for your helpful answer. We have analysed our servers and found the problem is happening on the server where there is heavy load so you are right at this point we need to think about Crystal Report Application Server.
Now suppose we have three servers and our infrastructure is physically 2-Tiers i.e application and database are hosted on the same server and in that case what type of Crystal Report Application Server we need that would suit our current infrastructure. Also we cannot consider web farms at present.
Thanks