Goes part way into boot up, then shuts down

So when it responds to pressing the power button at all, (it not always does) then it will chime, light up, go to Apple logo, progress bar about 1/3 of the way and shut down. I tried another known good battery which helped it boot up instead of just acting dead. It has a light on the mag. It does not respond to SMC reset, it will just boot 1/3 of the way…It does not comply with PRAM reset, ignores it, same with Com/S etc . I disconnected various peripherals, such battery indicator, DVD drive still no cigar. I also tried booting of external known good SSD, in case the SATA connector was damaged: same 1/3 boot. So the bug is in the pre OS loading aspect. This is not the first time I have come across such a machine, and really would like to find out where to look and what is the most likely culprit. …Any pointers from the gurus would be much appreciated. I have not tried another RAM chip yet, but will here shortly. Cheers, J