Two issues: White artifacts around edges of viewport, and flickering scene problem

[LaithArkham]

First problem (white artifacts):
http://postimage.org/image/sot7jsa0v/

- The viewport starts off like this when in the lit modes (not unlit or wireframe mode)
- The play-in-editor mode also starts off like this, but the problem can be fixed when using setres to shrink the window (for example, setting it to 800x600 makes it go away)
- After opening and closing the PIE, the editor viewport returns to normal (meaning the white spots go away) as long as the viewport stays small -- if I click Maximize Viewport, the issue returns. I can get it to go away again when I set the view back to 2x2 split
- The PIE continues to display this problem each time it is opened up until changing the resolution.
- The in-editor viewport seems to display this problem each time the program is started up.

This is the first time I've used UDK. For whatever it's worth, I've bee using the UT3 editor up until this point, and it hasn't displayed any issues like this.

Second issue:
http://postimage.org/image/jcf847uoh/

- When the default "daytime" map loads on launch, either having the "update in real time" button pressed, or moving the viewport through the world, causes the viewport to rapidly flicker between the image on the left and the image on the right.

Does anyone know what might cause these and what might be done to fix them? Any help would be greatly appreciated.

[reinrag]

Are you using a custom post process ? when i done my water effect post process i had the exact same issue, but this was due to my material,

[LaithArkham]

No. The first problem is shown on a simple "cube map" with only a subtractive BSP cube (inside an additive BSP cube) and one baked light. No materials, no post-process.
The second problem is on the default daytime map template that loads when I launch UDK.