My PC cannot start

[combine]

Hello, I have extremely strange problem. Yesterday I replaced my old CPU radiator with new one because of overheating. I disassembled my PC that I can work with the motherboard. When I put everything in place I tried to turn on my PC. It could not start but I changed the place of the RAM memory modules and then everything was OK. Today I woke up and I tried to start it on but with no luck. PC components work but nothing happens. After a lot of "playing" with it it booted up windows and I flashed my bios but that didn't work.
I figured out that when the PC case is in normal position it won't boot up, everything is working but there's no signs of life. If I lay the case down as in older computers, it is working at least for a while. Then after 5 minutes it turns off again(restarts). My memory is 4x1GB Adata 800MHz.
I noticed that with one memory module it works perfectly fine, but when I put two or more it do not work again. I've got NO idea what could cause the problem so please help. Thanks

 

[JobCreator]

List your specifics including mobo, hdd, psu, gpu, cpu, etc. "Laying it down" should have absolutely no effect on the stability/operation of the system. Are you using water cooling? Or do you refer to the heatsink as a radiator?

 

[Swordkd]

What cooler did you put on your Cpu? I would disassemble the pc again and double check all connections.

 

[Swordkd]

List your specifics including mobo, hdd, psu, gpu, cpu, etc. "Laying it down" should have absolutely no effect on the stability/operation of the system. Are you using water cooling? Or do you refer to the heatsink as a radiator?

It can if the cooler is too heavy for the motherboard. I.e., warping and causing a short or something.

 

[combine]

CPU - AMD Athlon 64 x2 5400+
RAM - 4x1GB Adata 800MHz
HDD - Western Digital 320GB
LG DVD-RW
PSU - Fortron 350W
VGA - Nvidia GT240 1G
MOBO - AsRock K10N780SLIX3-WiFi

No, I'm not using water cooling, I mounted CM hyper 212 evo.

It is working fine with one memory module, doesnt matter if it is laid down or straight.
Also I tried to put that RAM in another slot and it worked until I tried to put more than one memory.

 

[combine]

Also when my PC is not starting up, the DVD is struggling (looks like trying to spin the motor then after 1 sec it stops)

In the morning when this happened I randomly removed the lan card and PC started with no issues. When I got back home after several hours, I tried to turn it on but with no luck.

Update: I tried to remove the lan card but I didn't have this luck again.

 

[JobCreator]

List your specifics including mobo, hdd, psu, gpu, cpu, etc. "Laying it down" should have absolutely no effect on the stability/operation of the system. Are you using water cooling? Or do you refer to the heatsink as a radiator?

It can if the cooler is too heavy for the motherboard. I.e., warping and causing a short or something.

Well, given that everything is secured and seated properly, and assuming you don't have a mobo made of plastic wrap...

 

[combine]

What if the heat pipes of the heat sink contact slightly with the case? Could electricity somehow kill my mobo. Also when I returned the stock one nothing changed at all (problem unsolved).

 

[JobCreator]

What if the heat pipes of the heat sink contact slightly with the case? Could electricity somehow kill my mobo. Also when I returned the stock one nothing changed at all (problem unsolved).

There's no current running through the heatpipes, so it won't short out or anything. The fact that it works when laying it down is quite strange. Like previous posters, you might want to check that the mobo is properly seated in the case and the HS/fan are not succumbing to much to gravity, resulting in an unsettled mobo. That can surely cause shorts in the circuitry.