This is an old thread, but after reading all 17 pages (since I have the same issue), there still hasn't been a solution posted.
I had an M2 ATX setup that worked flawlessly for about a year. Then, I had some issues with my monitor. I waited about a month to get my new transflective screen (only change to the system), and now the computer won't post. It does the same thing as has been repeatedly been stated in this thread...twitches the fan every 5 seconds, but won't power up.
Using a desktop pc PSU, the board boots fine.
I have swapped out with a different motherboard: no post. I tried different memory: no post. Unplugging all components drawing power: no post. I even bought a new M2 ATX, thinking it was clearly the bad link: still no post! I tested my input voltages from the starter and from the car battery...both give a steady 14V. I tried switching the power cable (polarity), swapping the 20 pin connector between the mobo and psu, and repeatedly hitting the reset button: no post.
Does anyone know anything yet about this problem?