I would start by disconnecting all hardware from the computer (hard drive, all cards including your PCIe Video card, DVD rom etc etc etc).
Then with only one stick of memory and the video connected to the onboard connection, see if you can regularly boot up to the POST screen and enter the BIOS by pressing appropriate key at boot up (I think it is the DEL key on your system).
If it is still a problem, try swapping to the other stick of memory.
If it is OK and you can enter BIOS every time, then try connecting the hard drive with the one stick of memory (nothing else) and see what happens.
The results will determine your next steps.
Also you say that sometimes you give the parts a dust, have you tried purchasing a can of compressed air and blowing all memory, PCI slots etc out.