Dell P992, the sharp turns blurry!

Greetings.

I'm having some problems with my Dell P992 Trinitron monitor.

The trouble is that suddenly the whole screen turns blurry, so it's impossible to read the letters under the icons, look at pictured, or even play games, etc etc.

Since I'm playing alot on my computer, this little "error" is making me MAD! I've tried switching resolution and the frame rate. I've opened the computer and cleaned it from dust, to be sure the computer won't overheat.
I've even replaced my nVidia GeForce 4 Ti 4600 with a older nVidia GeForce 2, and still no difference, the screen is still blurry!

And of course, I have tried switching screens, and my brothers screen is working just fine on my computer, so I guess it has to be the P992 monitor... I'm not sure though, and I really wondering what I've done to "create" this major "error".

Another thing I've noticed too, is that sometimes the screen goes sharp again, and then turns blurry a few seconds after it goes sharp. Maybe the error is located in the cable which goes from P992 to the computer?

Well, I don't know how to fix this - I've tried every single way.. (I belive) - and the screen is still blurry!
And, since the computer is ~2.5 years old, my insurance is gone - and I really don't want to PAY for a NEW monitor -
a 2.5 years old monitor shall WORK, end of question.

So, could anyone give me some help here?

Regards,
Christer Fredholm