Terminal server bottleneck

Hey anyone

I am running a Win 2k server and two Win 2k terminal
servers connecting 15 stores via isdn routers and lines.
The remote sites access a common application and database
on the main Win 2k server through the terminal servers.
The problem is that when the traffic increases on the
network it slows down and the remote systems time out
which causes errors in the application. I have increased
memory in all the servers and performed all maint. etc.
but cannot get this working smooth. If anyone has a
suggestion I would appriciate it.

Thanks,
Don W.