Problem calling web service asynchronously

In my web service I have Method1.
When attaching the web service in web references, I can also access Method1Async.
I am using this Method1Async to call web service asynchronously.
The problem is - when I have many requests, I get this exception:
An established connection was aborted by the software in your host machine
With small number of requests it usually works.
3-5 requests - fine. More than 10 - problems.

I understand it is related to available threads in IIS or XP, and that number may vary all the time.
Is there a way to solve that problem?
In my web service, on a client side I see only 2 methods: Method1 & Method1Async.
Thanks.

...