|Resolution depends on your laptop screen size but 60Hz is typical for LCD's. This might help if you have no other information:|
EDIT: Seems your laptop is 1366x768.
Out of curiosity is it any different if you run the laptop without the external monitor plugged in?
The symptom is a bit unusual. My best guess is a hardware fault, such as the screen itself (inc inverter), or the connection between the laptop and the screen. Sometimes the cable gets damaged where it hinges back and forwards (check that nothing changes as you tilt the screen back and forth). One of the end connectors becoming loose is also possible.
I've rather precluded driver (software) issues because the external monitor is fine.
Always pop back and let us know the outcome - thanks
message edited by Derek