I hope the developers know about this.  

I can also confirm that this is a problem. 

From what I read on the NVidia web site, the level 340.52 GeForce driver has a lot off issues, not just with GC3, but with a lot of other games as well.

If anyone needs info on how to back off the NVIDIA GeForce GT series driver from the troublesome 340.52 level, here is what worked for me:

To roll back your driver (I recommend the 337.88 level) go to www.nvidia.com -> "Drivers" tab ->"Beta and older drivers", fill in the six fields (all of them are pull down lists) "Product type", "Product Series", "Product", "Operating System", "Language", and "Recommended/Beta" and click "search". Then select a driver to start the download.

Don't forget, if you are using "NVIDIA GeForce Experience" to change your settings to prevent automatic updates. Leave notifications on so you can get a notification of a newer driver when they fix this problem (which might initially be a Beta).

No, its a GT640.

The driver in question covers a lot of models: 8 series (8xxx), 9 series (9xxx), 100, 200, 300, 400, 500, 600, and 700 (including TITANs)

I'm on 340.52 with a GTX 780M and don't see these symptoms.  I tried running fullscreen at 1920x1200. windowed at the same resolution, and windowed at 1680x1050.

Jack, I assume from the screen shot you posted that this happens pretty much as soon as you start the game?  I played 4 or 5 turns, tried zooming way in and way out, and still didn't see the lines.

Yep. It showed the instant I started the game. It was so bad that I didn't try to do anything at all once the game displayed the main map. I also had this problem several weeks ago with the Beta driver and posted an issue here. In that thread I showed some of the debugging I tried and found that there were two game video options that would suppress this failure if they were both off. Otherwise, it hadn't mattered how far it was zoomed, in or out. Now I can't find that thread. It had some useful info in it.

Edit -- I just checked the video options for the game and the options that I had to turn off are "enable bloom" and "Enable ambient occlusion". Even with those options off, I would sometimes get the flicker Satoru1 describes. And, in the latest "Dev Stream" when they were demonstrating the effect of "bloom" I got the impression that the level of GC3 we have doesn't have anything in it to take advantage of "bloom", or at least not all of the code does.

Edit 2 -- I found my old post on this issue. Sometimes you have to be inventive with your search argument.

Edit 3 -- Here is a better thread with better descriptions.

There was a fix for this issue implemented into the Beta release. Please let me know if you see the issue after updating to the latest version of Galactic Civilizations 3.

I just re-installed the NVidia GPU driver level 340.52 and brought up my current game and the horizontal lines are gone, at least from what I can see with a very cursory check.