how far does it boot? how long do you have to wait for the flashing? if this happens just at start of booting you graphics card or monitor could be damaged, or your cable connection. does the kernel switch video modes at the beginning of boot (smaller font or graphics mode with little penguin)?

if it took 30 seconds or so before flashing, it may try to start X and the X server may have problems with the graphics card. you could start the system in single user mode (using boot command line parameter 'single') or from a live cd (like grml or knoppix) and configure the system to not start X (e.g. put an 'exit 0' at the beginning of /etc/init.d/[gkx]dm, whichever you use). i had fried mobo capacitors that couldn't deliver the current for cpu _and_ graphics card, and an overheated graphics card; both lead to only text mode working and the hardware failed when X tried to switch to graphics mode.