My Computer Comes Wierd

I read a lot of Threads and hoping i can see same as what i am experiencing right now. Im just a new member of this forum.

I have this Comp. Unit

- AsRock Wolfdale 1333 - 667
- Intel Dual Core 1.8Ghz CPU
- Intel® Graphics Media Accelerator 950(Max. shared memory 224MB)Built In
- 512MB DDR2 Memory Card
- 80 Gigz SATA HardDrive(Seagate)
- IDE ASUS DVD RW

I experiencing this sudden turn off after a couple of minutes then after i try to switch it on it boost again but in a seconds it turns off again. A lot of different situation is facing of my PC. Im playing Halo Game sometimes Fable and dota, my monitor turns no signal but my PC is still ON but no response with the (num-lock, caps,Scroll) light. And even a long press on the power switch, so i turn off the AVR.

Another is a Sudden display of lines but PC is on. Most of the time is a sudden turn off and no video signal. Since my PC is keep on turning off suddenly so i patiently trying my best to use my PC even it turns off. Another is this when it turns off, i switch it on again but a sec after pressing it turns off like a magic. a blink of power on and off again. then i wait for an hour then turn it again sometimes it back on but experience again the sudden off and often i wait for a day to until it turns on. Another is when i turn it on, no light from main and HDD but i hear the HDD is spinning and fan is on but no beep. What should i do? I went to a repair and check it. the PSU, Video, Memory. but they are good. they said my memory is a generic or no brand name in its chips does this affect my PC? And they use pin to test the PSU but its normal they say. but i using my pc for about a 1 year now. last Oct my PC experience this kind of problem. can anyone help me? my I already clean my PC but no trace of dusty surface. when i use 2 connected HDD SATA and IDE,the Primary is SATA, a sudden i can't access the IDE slave and disappear in my computer. If you have same kind of problem with me please share your ideas and solution for this. Until now it keeps on Turning off in a sudden.