I've been running into the same issue over the last month. I figured I'd pop on the forum and see what you guys think.
Many of our users have been issued Toshiba Tecra A10 laptops, complete with Toshiba Port Replicators.
The dual monitor trend is also catching on around the office. The problem lies in the fact that when connecting an external LCD to the docking stations, the external monitor flickers.
If I un-dock the laptop and directly connect the external LCD to the laptop VGA port, the image is crisp. Seemingly, the problem lies in the docking station. I've tried reinstalling drivers, changing the refresh rate, using a different monitor/brand and using a DVI connection instead of VGA but the problem still remains.
Any direction or suggestion would be much appreciated.
If you're using a port replicator I'm guessing it's connected to your laptop via USB. Assuming that is correct then I believe that's where the issue might lie - what you are trying to do is run video (and a whole lot of other things) via one shared USB connection to your laptop. Video can be bandwidth intensive - and that's the limitation of these port replicators when compared to a full/proper docking station.
I've just bought a netbook and my only option is to use a USB "docking station" - been doing a lot of reading and just about every review for any brand of USB docking station said the video was jerky but everything else was good. When you connect your monitor directly to the notebook you then bypass the USB limitation and hence you get a perfect VGA output.