E6400 won't POST

I have a recurrent problem with my E6400(basic config, P8400, 4GB RAM, HDD, ODD, Intel IGP and so on, running W7 Pro x64).
The problem is when I power it on I get black screen, after I few seconds I can hear sound from the ODD then the HDD then the fan goes to full speed for a several seconds then the laptop shuts off. The whole process takes around 20 seconds. There are no LED codes so I can't identify what the problem might be.
It might sound weird but this has happened before and every time I manage to fix it only I don't know how...
Here is the thing: when that happens what I normally do is disassemble part by part(everything, including the CMOS battery and the CPU) and try to start the laptop after each removal then the same thing but backwards and when all components are back together then eventually at some point the laptop starts normally. Every unsuccessful attempt to start it ends by the laptop doing the same thing described above, only when I take out the CPU I get LED code for missing CPU, but with other components removed I get no codes, nothing.
So I'm thinking bad CPU or mobo but if that was the case how come the laptop eventually start normally and work for several weeks/months? Any ideas?

P.S. I forgot to mention that there are no usual activity performed on the laptop prior to the problem, no BIOS flashing or OS installing or unexpected shutdown on anything like that, I just shut it down myself after normal usage then it won't start properly...

P.S.S. It seems that even if the laptop fails to boot the BIOS is working because when I finally manage to start it normally when I check the event log of the BIOS I can see some recorded logs about the CPU being missing(coinciding with the the time when I had the CPU removed so it's not a real error).

 

I've seen this on a couple of E6400/E6500 motherboards, and as far as I can tell it's some weird motherboard failure; ie. I've never been able to fix it by replacing components other than the motherboard.

 

Thanks, that's what I'm thinking too. Any idea if I have to get a motherboard with the same part number or any E6400 board with the same layout as mine(and with Intel IGP) would do?

 

Well i think you would need to get a different heatpipe assembly as well though I am not sure.

I would say your best bet would be to buy the board with the lower assembly. They seem cheap enough for a tested board, palmrest, heatsink etc.

Dell Latitude E6400 Laptop Motherboard Base Palmrest G637N Tested 4053162046498 | eBay

 

All of the Intel motherboards use the same heatsink and all of the Nvidia motherboards use the same heatsink. You only need to change heatsinks when going Intel to Nvidia. The Nvidia heatsink works on the Intel motherboards. The palmrest is also slightly different between Intel and Nvidia boards; as far as I know you'll generally have to change that going both ways.

 

I must be lacking sleep I thought he was suggesting a move to Nvidia and what that would entail