Connect a system speaker because its diagnostic beeps can be very helpful.
You're going through a lot of trouble by removing just one RAM chip, which requires unsoldering it. Wouldn't it be a lot easier to deal with whole RAM modules instead?
I'd disconnect the power from the hard disk because there's no need for it during testing.
The power shutting down in seconds indicates a short, but some motherboards will shut down if they detect an overheated CPU (often from poor contact with the heatsink) or stalled CPU fan. The most common shorts are between the motherboard and case, usually around one of the mounting holes (electrically insulate with fiber washers on top or bottom, as needed) or at a corner (attach rubber stick-on bumper to case there). Was the motherboard installed over brass standoffs, rather than directly against the case? Almost all cases require those standoffs. However you don't want any extra standoffs under the motherboard because they'll short to it or even crush tiny components there.
While you have the motherboard removed, look at the solder joints of the DIMM sockets with a magnifying glass and strong light because one may be cracked. It can be fixed with a 40 watt iron and some rosin flux. A less powerful iron probably won't help and may actually cause more heat damage because it won't melt the solder quickly. YouTube has some soldering tutorials.
Check inside the DIMM sockets for bent contacts that may be shorting together or any debris in there. Hold the motherboard upside down and tap on the back to remove debris.
Have you tried the memory modules in another motherboard? If that's not possible, get 8 spring-loaded clothespins and attach one over the middle of each RAM chip. If that makes the problem disappear, most likely a RAM chip came unsoldered, probably thanks to a heatsink attached to it with double-stick tape (pulls chips when module is pressed into socket).
