Go into Control Panel, Display and set the refresh rate and (perhaps) resolution to the screen's native settings. LCD's are usually happy at 60Hz.
Then select the resolution which fits your needs (and, hopefully, the screen).
If still problems, look for a newer graphics driver at Dell site.
The refresh rate is already at 60Hz and is unchangable, so newer graphics driver could be the key.
thankyou very much for your response and help.