Dell Inspiron 6400/E1505 randomly won't boot

Hello,

I've got a Dell Inspiron 6400 (E1505) that has issues.


It seems to run just fine for a long time (weeks to months of normal use) and suddenly and randomly the screen will go black, sound will cut out, and nothing can be done to change it (no exporting screen image, no sleeping/hibernating) and I have to manually shut it down.


After doing this there is about a 30-40% chance that when I go to turn it on again, it will not. When I press the power button, I get four lights coming on-- the power LED in the top right corner, the capslock, numlock, and scrolllock
LEDs in the top center. They light green for about three or four seconds (solid, no blinking) and then turn off. Nothing else about the computer turns on (no fan, no HDD, etc.). If I plug in the battery, it will charge. If I hold down the power button, it will cycle through this on/off routine in the LEDs incessantly. This happens with battery and AC together, just AC, or just battery.


The thing is, eventually it will turn back on. Sometimes if I leave the button pressed in under a weight overnight or for a couple days, it eventually turns on and runs normally for several days, even through booting/hibernation/sleep cycles. I have no idea what causes it or what causes the fixes. The fixes I've tried seem to be random, sometimes helping, sometimes not.

In the past I have tried:

*unplugging the AC and de-latching the battery, holding power button for 30-60 seconds to drain flea power, then re-plugging everything and booting. This has worked before. It currently does not.

*reseating or replacing memory

*draining the battery by constantly cycling the LED routine with a weight on the power button like above, then plugging in once drained.

*reseating the CPU and cleaning with compressed air (just tried this two weeks ago and it worked; it was the most recent time the blackout happened before now)



These have worked sometimes, albeit randomly, in the past, but currently it is blacked-out again and nothing seems to work. So far I have tried:

*reseating each RAM module, seating one at a time, and seating each in the opposite slot to test slots/cards separately.

*replacing the CMOS battery.

*Discharging the flea power after removing battery/AC

*opening everything, detaching everything I could find/think of from the mobo and re-attaching, cleaning a bit (though I just cleaned thoroughly two weeks ago) and reseating the CPU.


Nothing so far has worked. It's been off for two days now. I'm currently leaving a weight on the power button in hopes it will turn on sometime tonight. I am skeptical about certain hardware issues because when it actually DOES turn on, it runs fine. Any ideas? This has been plaguing my computer for about a year or more, but generally it fails only every few months. Lately it's gone out a few times in the past few months--would really like to fix!


Also, my AC adapter LED lights up when plugged in. Any help is greatly appreciated; sorry for the long post; wanted to be as thorough as possible.