After I overclocked my graphics card, my computer has twice showed a light blue screen with thin, light purple lines running vertically across the screen. The first time it happened, it came with a clicking sound, as if something that was supposed to be spinning was stuck. I asked one of my friends about it and he suspected that the graphics card overheated. I lowered the clock speed of the GPU by 30Mhz, and it was fine for a while. But a few minutes ago, it happened again minus the clicking sound. I would like to know if anything is wrong with my computer and how to fix it without lowering the clock speed of my GPU. I would upload a screenshot but the computer freezes whenever it happens. I am using a Windows 7 Ultimate 64-bit on a Dell XPS with an ATI Radeon HD 3820 graphics card overclocked from 777Mhz to 855Mhz, and an Intel 2 Quad-core processor running at 2.66Ghz.