Just in case anyone else suffers from the same problem that I have been having:
After installing the latest official wheezy raspbian image onto an SD card my latest Raspberry Pi would only allow me to run the raspi-config for the first time. After configuring this and rebooting the Pi would hang on the rainbow splash screen. If I unplugged the Raspberry Pi for 10 minutes it was possible to boot up. I tried three different SD cards and all suffered from the same issue. I didn't have this issue with my first Raspberry Pi.....
I kind of got lost in the discussions about SD cards and power supplies but the fix I found was to add the following line to the config.txt:
The raspberry Pi now boots first time every time on the three different SD cards that I tried using.