Few days ago I had ASUS A8N-E s754

Few days ago I had ASUS A8N-E s754 with same symptoms. First start normal with message that CMOS is cleared and need to run SETUP or LOAD DEFAULTS. 

After changing BIOS version and trie with another BIOS chip in desper I found that supply of NF3 chipset is slightly higher then normal (about 0.3V higher). After changing bouth transistors in Chipset supply the voltage vas 0.3V lower then normal and symptoms was still present.

After changing another set of transistors the voltage was exactly as it should be:) Evidently second pair of transistors was also defect and third pair was proper.

After that, Mamka started to turn on normaly.  Maybe you should try another transistor(s) or another model of transistor(s)?