Inspiron 620 - New Power Supply, On/Off Switch, New Motherboard, will not start

OK, I believe I am somewhere between Geek Squad and professional computer fixer.  Dell Inspiron 620 would just quit.  Never while using it, would come to find it dead.  Unplug power and everything else, let it sit for hours or days, it would come back fine.  Finally died.  I know, should have done more troubleshooting, please don't tell me the obvious.  Finally died.  Would do nothing when power button pushed.  Happened about the same time the wife's office computer died (yes, I am tech support for her too, but she still have warranty).  I ordered an exact replacement power supply and power button (since I found a number of threads about the POS power button on 620 desktops).  Replaced them both.  Still same symptoms.  NOTHING happens when you push power button.  So I found a link about testing power supply, by jumping the green power and black ground.  Old power supply seems to be fine.  So since new power supply and power button did not fix the problem, I ordered a replacement mother board.  It seems to be the same mother board, based on order info and labels on motherboard.  now the PC powers up, gets to the "Starting Windows" screen, then freezes.  I did the PF12 boot options, selected diagnostics.  Ran through the diagnostics, it ran for over 1 hour with hard drive and memory checks not finished.  Cancelled, restarted just hard drive check, it ran fine.  Then started memory check.  It took over 6 hours to complete, but finally finished.  Still won't boot.  Gets to "Starting Windows" screen and finally dies.  Only thing plugged in is monitor, keyboard and mouse.  .  So I went and found the PC recovery disk I created when I bought the PC.  Booted from that, gave me the option to start normally or do recovery.  I selected do recovery, since normal boot does not work.  It ran for several hours, then told me recovery could not fix the problem (no, I did not write down the exact message), then asked me if I wanted to go to a restore point.  I said yes, then 7 hours later I finally turned the stupid thing off.  I am tired of throwing parts at this, don't want a new computer, does anyone have any ideas?  I had not installed anything new (hardware or software) for several months.  Home computer used for email, web surfing, Quickbooks for wife's business.  We are adults, don't surf all of the typical malware sites...