When you connect to the external monitor are you ... 1. Turning off the laptop, then connecting the external monitor, turning on the monitor, and then turning on the laptop? In that order. 2. What did it do when you do hooked up the external monitor? Was it the same problem, or a different one on the external one?
If the same exact problem occurred on the external monitor, then first I would update the drivers. If that doesn't help, then you may need to check the graphics card, or even replace it.