Multiplayer problem.

I cant get the atc controller option to work. Every time I choose this option whether it's hosting or joining an existing session, I get as far as the loading scenery objects bar and the sim freezes. Checking task manager at that point reveals that FSX is not responding and I have to select end program to get my 'puter back. Any ideas where I can start looking for the cure? I should say it works ok If I choose to fly instead. My specs are intel p4 prescott HT 2.8 cpu, 1gig ddr ram, radeon 1650 512 mg. graphics,and soundblaster audigy.