Monitor loses signal while playing game

Hello guys,

I have an ASUS P5Q Deluxe with Intel Core 2 Quad Q6600, 2GB of RAM DDR2 and Seagate 500GB. Before, I have NVIDIA 9800GTX 512MB and now I am using an NVIDIA GTS 250. My problem is, I can play games such as Mafia, Crysis, Hitman, Dota and more after few minutes, the monitor will suddenly lose the signal. I can still hear sounds coming from it.

I will then restart my computer and then play other games like Fifa, PES and NBA 2012. However, playing those games for some time, the same issue occurs. I decided to bring my computer to a technician but they couldn't find anything. After few times of bringing my computer to the repair shop, they offer a different video card which is NVIDIA 250 GTS. But it still didn't fix it. I tried everything and even changed my PSU from ELT500 Enrmax to 750W Energon. I don't want to change my motherboard so if there is anything that I could do to fix this, please let me know. I am really desperate to fix this. Thank you.