Am I reading this right: you used to be able to display at this resolution, and now you can't? I would do a full hardware check. Is the video card overheating, under-powered, not well-seated?
Edit: If your video comes directly off the motherboard (no "video card") please ignore this whole post.
- shut down PC and remove power cord to the PC's power supply
- open up the case, maybe take it outside and spray out the dust bunnies with some canned air
- an air compressor works even better...but: DON'T SPIN THE FANS (you'll wear the bearings out) . . . DO NOT USE A VACUUM CLEANER
- find the video card and make sure the fans/heatsinks aren't gunked up with dust and fibers
- unsecure any latches or anything holding it in place and remove it, then put it back in (checking that it is "well seated")
- if power is pulled from the power supply (as opposed to directly off the motherboard) make sure the right things are plugged in
- take your clean PC back inside and plug everything back...sometimes an unplug/replug, unseat/reseat "magically" fixes mystery problems