When Mac is on, network is messed up?

A customer called, said that when their Mac is turned on (OS X), other PCs on the network cannot access mail and the Internet gets slow.  I noticed that there was some delay on the Mac sending e-mails with attachments.  Earlier in the week, the router an modem were replaced and the problem seemed to go away with that, however I'm told the problem is back and they're noticing it when the Mac is on, and that the problem goes away when it's off.

I'm not sure if that sounds far-fetched, but do you have any thoughts on it?  The Mac is using Mail for the e-mail, and a built-in wireless card for the Internet.
christo87Asked:
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Vee_ModCommented:
Closed, 500 points refunded.
Vee_Mod
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TheTechGuysNYCCommented:
Seriously doubt that it is the Mac that is causing this issue. I have never seen such a thing...

... unless the mac has a ::cough:: virus, which is highly unlikely, but possible.

Maybe the Mac is running some file sharing software or trying to update several programs....

I need some more information.
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dalesitCommented:
What are the IP addresses of the Mac, and the router etc.? Are network addresses assigned via DHCP? Where are you getting your email from? A server on your network or from your ISP? Do you have a single shared account, or do you have multiple email accounts? How frequently do your systems get mail? Is the problem there when the Mac is on, but running no programs, or when it is running Mail?

Are all the machines connected wirelessly, or wired, or a mixture of the two? What does the router think is happening over the connection when you are experiencing the problem?

Cheers,

Joel
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strungCommented:
Sounds to me like an IP address confict. Make sure the Mac is set to DHCP.
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mischa_Commented:
Try little snatch at http://www.obdev.at/products/littlesnitch/index.html
Then you can test the outgoing internet connection!  Also try a wired internet connection, maybe the wireless card is malfunctioning...
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christo87Author Commented:
Turns out it was interference.  Very strange, because for over a year there were no problems (and nothing has been changed in the house/network).  So, the Mac was put on a wired connection, then we were noticing problems when the phone rang, but the phone is 5.8 GHz.  Weird.  The router was moved and the problems went away.
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mischa_Commented:
If you have a fairly decent router you can even set the frequency to an other level/range of GHz.
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christo87Author Commented:
Once the router was moved, the problem was solved.  It's a WRT54G, but I didn't put DD-WRT on it.  If moving the router still didn't solve the problem, I was going to attempt to put DD-WRT on it (so long as any interference didn't come in and knock me off - nice, interference and a brick :) ) to up the Xmit power to try to overcome the interference.

I did it at a friend's house, now they go from offline when the microwave is on to slow browsing.  At least they stay on.
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