Unknown problem. Please help!

I built a system for a friend of mine recently. I had it at my house for a good week, installing software and testing everything to make sure it was all solid. I delivered it to him and everything was fine for another two weeks. Then he got cable internet and we ran into some problems, ended up reinstalling Windows XP Pro. Got the problem sorted out (was a networking problem), and everything was fine again. Today he installed a wireless pci adapter and some odd things started to happen. I'm not saying the wireless card has anything to do with it, but wanted to mention it. Anyway, what happens is suddenly and randomly he will get repeating horizontal lines across the whole screen - see image below. It could be the monitor, which is an older eMachine CRT monitor. I'm really hoping its not the onboard video. Please take a look at the pic and tell me what you think. System specs:

Biostar NF61V Micro AM2 (GeForce 6100)
AMD Athlon 64 X2 5000+
1GB G. Skill DDR2 800
320GB Western Digital SATA HDD
Linksys (model #?) PCI Wireless Adapter



Thanks in advance for any help.

Id say the monitor is crapping out.

That looks to me like some sort of magnetic interference.

Are there unshielded speakers nearby?
Does the monitor have a good cable with a magnetic choke?
Is it possible that the pci card is transmitting something?
Is there something with a motor nearby turning on intermittenly?
(a window a/c unit for example?)

Test the system with a lcd monitor.
Test the crt on another system.
Will degaussing the crt monitor do anything?

If all else fails, it's probably time to go LCD anyway.