You need to eliminate each part of the system.
Try a new SD card from the list of supported peripherals sd cards
Test the power being supplied
Please unplug everything and check the power test points.
Its also worth noting that USB power plugs often fail to supply the require 5v EVEN if they are labelled to do such.
My own experiments showed that a difference of just 0.15v(avg) was enough to cause sporadic stability issues, that would affect the image on the SD card.
Find another Raspberry Pi user and test your SD card in their system.
When you do find out what was causing the issue please come back and comment so others can benefit.