X360 & Samsung SM 930bf Problem

This is one annoying problem. I ordered a Joytech VGA Cable which has arrived today. How all giddy I became! But there is a problem!

When I connect the cable to the X360 and into my monitor (Samsung SyncMaster 930bf), I don't get an image. I get sound which is through an optical cable into my speakers, but I get no image.

Oh, wait. I have just got an image in 640x480... ah its gone again. In 640x480 I get an image, but it keeps cutting out as though it is losing signal and then regaining signal. Now VGA mode works with my monitor as that is what I'm using at the minute to test. I don't know if anyone else has heard of any problems like this? I tried ringing Samsung and they said:

"Well it's either a problem with the cable or a problem with the monitor."

Joytech couldn't really help me but they said they are going to "investigate" it. If any of you guys can help me I'd really appreciate it!

Btw, when I try selecting other resolutions from the X360 menu (when I can see the list), they don't work. Selecting 1280x1024 just gives me an awful image with everything off the screen as though the signal is out of range!

Thanks in advance, James.