External display problem

Hello, i've got a problem when connecting external LCD widescreen display to my macbook pro via mini display-DVI connector. When I first connected the LCD monitor everything worked just fine. I turned mirroring mode on, adjusted resolution from 1280x800 (macbook native) to 1600x1000. After installation of the insomniaX 1.3.2 program package i restarted the computer. Since that time i am unable to set up resolution higher than 1280x800 in mirroring mode. When i switch the mirroring mode off, the higher resolution is accepted. Do you know, where is the problem? Thank you very much indeed.