2407WFP VGA resolution problems

Hi there, I recently had my broken 2405FPW replaced with a 2407WFP. It's a great monitor, as I can have my PS3 plugged in via DVI @ 1080p and my PC via VGA. Now I should be able to get 1920x1200 via VGA with no problems - it's worked some of the time, and it worked all of the time on my old 2405. But now it's struggling to output at the native 1920x1200 via VGA. Sometimes it randomly decides it can't do it, claiming the max resolution is 1600x1200. Sometimes I've had the screen running at 1920x1200 all day with no issues, and then after a restart, it comes back at 1600x1200. In addition, I watch the ATI drivers, and sometimes it says 'Default VGA monitor' and then it flickers or goes black for a second and says 'Dell 2407'. And within Device Manager, it sometimes says 'Generic Monitor' and sometimes, 'Dell 2407'. I've tried downloading the 2407 drivers from Dell's site, but Vista claims they'r'e already installed (and it should be plug and play). I shouldn't have to mess about like this. Is it a dodgy VGA cable? Or is my monitor faulty?