No signal on monitor

good day, before i start i would like to say english is not my native language so i might make some mistakes

4 months ago i tried to turn on my computer everything seemed fine but when i tried to turn on the monitor the screen would go on,but i did not get a signal to my screen,also the gpu fan ran at 100% every time i started the computer. i thought maybe the cable that goes from my gpu to my screen was broken so i changed it but the problem was still there.
so i tested with a different video card and i got a signal and it worked fine so i thought it had to be my video card that was causing this problem(it was an old nvidia8800gt)

so i got a new motherboard MSi p67a-c43
new 2x 4gig memory
and a new gpu ati 6950 twin frozr ii
the only thing i did not replace was my psu 600w because it was only 1 year old.
so it all worked fine for 4 months until now, i get the exact same problem again no signal on the monitor and the gpu runs on 100% speed.
i really don't know what to do at this point or what could cause this problem again.
some things that i think that could have caused it but I'm not sure.
since i build this new computer sometimes i would get a blue screen saying SYS_memory I'm not exactly sure about the exact message sometimes it was different, but it always started with sys_memory. this happened about 5 times.

something else that might have caused it is that my monitor is kind of old and when i want to turn it on
i had to hold the power switch like 30 seconds and the led would flash and eventually it would go on,could it be this is bad for my gpu or something?
a friend of mine also told me it could be my video card overheating but i never had any overheating problem with this new card,i don't overclock or anything like that it always worked fine without any problems

the problem is i threw away my old gpu ram etc so i can't really test anything(stupid i know)
so if anyone has any ideas or can help me with this problem so i can prevent it from happening again that would
be great

Does this happen when it goes into sleep mode or standby mode? Or your not getting any signal at all?

Have you tried another monitor? Maybe barrow one?

Have you tested the memory? Memtest86 is the best.

What brand and model number is the PSU?

Edit: Also, what was your first GPU before you replaced it with the 6950?

Sorry for asking the wrong question. I was actually wondering what video card he tested the system with. From what I understand, there have been three video cards in play if I read correctly.

So it's not the monitor, and it's not the video card, and the only thing you didn't replace was the power supply, then it's gotta be the power supply.