Dell Vostro 260 will not boot consistantly

We have two identical Dell Vostro 260 towers that will not power up. It was thought there was a bad power switch on one PC because it appeared to the user that, if it pressed enough times, the PC would eventually boot.  It was as if on/off switch is broken..no fans, HD, beep codes, etc. - swapping out the switch did not resolve.  There is always a steady green light on power supply and a steady green light on motherboard.  If the PC is unplugged for a period of time...it will boot with no issue and run for weeks.  PC will power down and up, can be unplugged and plugged back in, will boot repeatedly with no errors and appear “fixed.”  Trouble shooting steps:

•  Verify warranty has expired
• Replace motherboard battery
• Reseat all connections/RAM inside case
• Swap with new (known good) power supply
• Boot with various peripherials diconnected
• Plug and unplug power until bored
• Press and hold power button (20 seconds or until finger gets numb) – Fans never run, orange light on MB never goes out.
• Wait until orange light on MB goes out on its own
• Search the forums…

 Assuming both PCs were beyond repair they were placed on shelf for parts.  After several months passed, idle curiosity set in and both units booted with no issue and worked fine until placed with end user at which time they failed to boot.  Both units, at this point, could not be booted. 

 Went back and again tried the suggestion of : Unplug the cord from the power supply, hold the power button in for about 15/20 seconds, open the case, unplug the 24-pin power connector from the motherboard and jump the Green wire to one of the Black wires, reconnect the power cord and power the system on, if power supply and case fans and the hard drive run, then the PSU should be good.  The result was pressing on/off button and there was no boot HOWEVER pressing the on/off button and HOLDING at least three full seconds - the system boots. The error is intermittant, always comes back, could be a low level code bug that senses a condition, possibly a marginal component or a just a disturbance in in the force. 

 Short version:  Faulty hardware can’t be identified.  Although both PCs can boot, neither Dell Vostro 260 can be depended upon to boot consistantly for end users.  Replaced with new PCs.  Still perplexed.  Related posts:

 http://en.community.dell.com/support-forums/desktop/f/3514/t/19373690.aspx

http://en.community.dell.com/support-forums/desktop/f/3514/t/19373690.aspx