Hello and welcome to Bleepingcomputer!
I read your entire post with interest because you gave a very discriptive explaination.
I see that you have been working on a RAM situation, but the real question is...is it really a RAM problem?
You should investiagte your video card as a possible suspect, if it is an expansion card you should try reseating it in its slot, it could be causing issues.
You could be having problems with your power supply as well, it might not be putting out the power it used to and this could be causing your computer to crash.
Make sure your processor is not over heating, this will cause crashes as well, you may need to clean the heat sink fins out and add new thermal paste in between the heat sink and processor.
Once you have done this try running the diagnostic software below if you can get windows to boot properly again.
Try speed fan at http://www.almico.com/sfdownload.php
or HWMonitor at http://www.cpuid.com/hwmonitor.php
Both of those programs monitor both voltages and temps from your motherboard monitors.
Make sure the RAM you are using is the correct RAM for your system, just because it fits does not mean it's the correct type.
I am not saying this is the case, but your motherboards capacitors could be drying up and causing problems, check those for leakage on their top and on the motherboard.
Please post back what you were able to try and what the results were and keep us informed.
Good luck and kind regards.