the fact you can choose the resolution you want is good. Sounds like the GPU drivers
are working correctly. Did Device Manager ever recognize the monitor by name? Or was it always generic PNP monitor? Does the monitor wake as soon as you turn the PC on? Your PC is acting like it doesn't see the monitor when it boots, and sets the default resolution. You are able to set it correctly because it is recognized as a resolution the monitor accepts.
You might try uninstalling the GPU driver
, running drivesweeper, and seeing if windows identifies the monitor natively. If not, install the latest ATI drivers. A Guy