I also have that problem but not every single time. After opening the phone the screen immediately goes blank and then reappears with parts of the screen blanked out. I have to press either home or end in order for it to refresh and appear again. It's kind of pain since you loose whatever application you had on at the time.

It seems like that the processor goes to sleep after a while and then when you open the flip it is slow to wake up and garbles the screen information.

I have a 256 MB SD card installed but no hacks or other background programs installed.