Resolution problem on external display on Macbook Pro (15-inch, Mid 2010)

Hi there guys. I recently bought a 23inch Dell Ultrasharp display for my 15inch Macbook pro.

Max resolution I can get from the screen is 1080p. Althought, I've noticed a weird thing.

After using my screen for like 2 and a half weeks, when I started my Macbook today the external monitor had a fixed 600x800 resolution which I couldn't change from the display menu on system prefs. Actually, I don't even get any resolution options when I choose the scaled radio button. So I can't do anything.

I can't reset pram cause obviously command+option+p+r don't work regardless the way I press them (i just get the recovery screen).

And its the same thing on windows as well (using bootcamp), fixed resolution and no option to change it.

Thing is that I updated my Macbook's ram from 4 to 8gigs from crucial. At first everything was fine but again after a restart this occured again.

I opened the case and used one 4gig from crucial and the default 2gig from apple. It worked for the screen and I got back to max resolution.

Then put the 8gigs again, worked fine for 2.5 weeks and now this again.

Do you think that this has anything to do with ram or its just Mountain Lion?

Looking forward for your help!!!

Many Thanks, Thanos.

(forgot to mention, I'm using a minidisplay to display port cable)