Hi Andrew,
The new monitor will have its own EDID which the computer does not know until it is plugged in.
The computer through its graphics card will then communicate with the monitor via EDID to determine a native resolution of the monitor. Fine if it can do it and if not then it will re negotiate a resolution supported by both. This may be what you are experiencing.
I think in your case, the EDID of the new monitor or 'plug and play driver' is not quite correct and therefore not allowing higher resolution when used as a stand alone monitor.
When you plug in an external monitor, the system may use the external monitor's EDID and then that may explain why your new monitor works in conjunction as its using a different EDID to what it has.
It sounds as though the driver to talk to your monitor is a default Windows one and when it talked first time to the new monitor, it did not get it correct.
I've just had a look at my work PC and when looking at the 'Windows' display settings, the display has the exact model of the monitor that's plugged into it and therefore has the correct EDID. This was a result of using the supplied disk that came with the monitor. Otherwise it shows up as 'Plug n Play'. Same for my computer at home, I build them myself and load everything.
Let's say your new monitor came with a disk. If so, after you install it, you would see the display listed as the model of the display and not 'plug and play'.
That would mean that Windows would be using the new monitors driver to talk to your new monitor and not the generic Windows one.
There are a couple of EDID reader programs out there. I found this freeware one, but I'm sure there are others.
http://www.nirsoft.net/utils/monitorinfoview.zip
I mentioned to update the drivers for the graphic card if possible as that can sometimes help as well as they all interact with each other. Can't hurt.
Either way, enjoy some beer!
Cheers,
Damien.