I had similar problems but then I manually set it right with the following instructions:
HOWTO: Wrong/Low Resolution and flicker
The Xorg server - the code that provides a graphic display - is very good about determining what an attached monitor can do. There are limitations.

This HOWTO explains one process for obtaining a rock-solid graphics screen within the constraints of your hardware's abilities. There are many other ways to do this.

Overview of the problem:
When Xorg starts, it tries to figure out what kind of monitor you have via DDC/CI and EDID protocols. If your monitor and cabling are first-rate, you will never need to care about any of this. (Yay!)

If not, Xorg will pick a "safe" VESA resolution and use that: probably not at all what you expected, and not likely to be satisfactory. (Grr.)

Lucky for us all, Xorg is highly configurable and it's very easy to get the exact resolution(s) that you want, deserve, and yes, paid-for with your invested time.
Warning: If you are terrified by a Linux command line, at least TRY to follow this procedure. It won't hurt you and it will help.

Still with me? Then let's proceed:


Step #1: What kind of monitor do you have? What are its capabilities? What is its maximum resolution?
Virtually every monitor vendor will publish the specifications for their products on the web. You can find them if you just look but first, you'll need the Vendor Name and Model of that monitor. It will be printed on a plate or tag, glued, screwed, or riveted to the back, top, bottom, or side of your monitor. Go look for it now. Write that information down and keep it handy.
Step #2: Fetch your specs.
Bring up a browser window and search for that model eg, 'dell 3007wfphc'.

Look for any site with details and specifications; the vendor's site is best, but most review sites will provide specifications too.

Ideally, you will find the "Horizontal Sync" range, expressed in kilohertz (KHz or khz), and the "Vertical Refresh" range, expressed in hertz (Hz).

If you found that, write it down like:
Code:
HorizSync 49.31 - 98.71
VertRefresh 60
and proceed to Step #4.

If not, then look for the "Maximum Resolution". This will be listed as something like "2560 x 1600 at 60Hz". Write that down.
Step #3: Calculate the frequencies from the specified resolution.
Open a terminal window (Applications -> Accessories -> Terminal) and enter
Code:
cvt 2560 1600 60
Hit Enter. This will output the following
Code:
# 2560x1600 59.99 Hz (CVT 4.10MA) hsync: 99.46 kHz; pclk: 348.50 MHz
Modeline "2560x1600_60.00"  348.50  2560 2760 3032 3504  1600 1603 1609 1658 -hsync +vsync
Aha! The vertical refresh will be exactly 59.99Hz and the horizontal sync will be 99.46KHz. Write it down as:

Code:
HorizSync 99.46
VertRefresh 59.99
Step #4: Create a basic xorg.conf
Write the following commands down on paper because we're going to turn off the GUI and run a few commands from the console.

Flip to the text console via Ctrl+Alt+F1 and log in.

Execute these commands:
Code:
sudo /etc/init.d/gdm stop
sudo X -configure
sudo /etc/init.d/gdm start ; exit
(ignore any notices about the "new" way of starting/stopping services - those "new" ways work, and so does this way, but this way works on all releases)

You're back at the GUI login screen, so log in and bring up the rest of these directions.
Step #5: Edit the basic configuration.
Edit the new configuration by opening a terminal window and running the following command:
Code:
sudo gedit xorg.conf.new
Scroll down to the entry
Code:
Section "Monitor"
        Identifier   "Monitor0"
        VendorName   "Monitor Vendor"
        ModelName    "Monitor Model"
EndSection
Add lines after the Identifier label to include the values you found (or calculated). The entry for that Dell monitor will look like:
Code:
Section "Monitor"
        Identifier   "Monitor0"
        HorizSync    49.31 - 98.71
        VertRefresh  60.0
        VendorName   "Monitor Vendor"
        ModelName    "Monitor Model"
EndSection
Note: If you appreciate details, fill in the VendorName and ModelName values (within quotes) like
Code:
Section "Monitor"
        Identifier   "Monitor0"
        HorizSync    49.31 - 98.71
        VertRefresh  60.0
        VendorName   "Dell"
        ModelName    "3007WFP-HC"
EndSection
Step #6: Save the new configuration.
Save that file. Then click File -> Save As (in gedit) and save the file as /etc/X11/xorg.conf then exit gedit.

Now, you have a new Xorg configuration installed where Xorg can see it, and you have a backup in your home folder.
Step #7: Restart X and test.
In the terminal window, enter
Code:
sudo /etc/init.d/gdm restart
You'll be logged off and the login screen should appear a few seconds later at full resolution. Congratulations, you have just done what your graphics card tried to do, but failed.
------

YIKES: If the screen goes black and stays that way, then you have either not provided the correct information for your monitor or your graphics card cannot run that monitor at these frequencies. Here's how you get back to low-res graphics mode:

Flip to the console via Ctrl+Alt+F1 and log in. Run these commands:
Code:
sudo rm -f /etc/X11/xorg.conf
sudo /etc/init.d/gdm restart ; exit
Now you can start over from Step #1, and find YOUR monitor's specifications. If you're very sure you have the correct numbers, then it is possible that your graphics card can't operate at the required frequencies. Replace it.