Black screen o pain

Hi man im getting desperate i have the exact same problem and i have tried resetting bios with the battery thing ,removing the ram, unplugging the unnecesary stuf ,taking the cpu owt and taking the gfx card owt and to no avail it still wont work...... when i power up all fans spin but the gfx card soes something weird.. the fan keeps reving up and then dow and up and down BUT when i removed the ram to test that other theory the gfx card's fan kept at a normal speed constantly, everything seems to work but still a black screen i have a brand new lcd that says "check siginal cable" but everything is plugd in when i removed the hdd and started no beep, removed the ram no beep this is the second time this year this has hapend and the pc is only 7 months old the first time i sent it in to the place where i bought it and they said the mobo pci-e slot is faulty and the gave me a brand new mobo now today a month later the problem is back with a vengence and i am really fed up plz any help or hints would be grately appreciated and one last thing when there is no onboard sadly

specs:
PROC-AMD 6000+~3.1Ghz
MOBO-ASUS M2N68
RAM-2X 1GB INFINITY DDRII
PSU-COOLERMASTER 550W
GFX- nVidia MSI 9800GTX+ PLUS OC
HDD-SEAGATE 160GB