UE4 freeze my pc

Hi, sorry for my bad english, i have problem with unreal engine because cause freeze to my pc. I checked the temperature with HWMonitor and MSI Afterburner before freeze, but I do not think this is the cause because my GPU (AMD Radeon 9750 (drivers are updated to the last version)) reaches 50/60° and my Cpu (Intel i5 750) reaches 60/70°… I use ue4 by an year, and this problem I have it from the last two weeks. The Engine version is indifferrent because i tried all version…if I use only the screen of blueprint,i havent problem, but when i tried a games with: “simulate”, “standalone game” ecc, after 20/70 seconds appears a screen like this: http://oi49.tinypic.com/34qpsnr.jpg and i must restart my pc. Sometimes also when i build the light or a project but only if I’m using the engine from one or two hours. If i open one of your project like Infiltrator, Reflection ecc, freeze after 2/5 minutes…In the past I never had these problems… My motherboard is an Asus p7p55D and the Ram is 4 gb.
I really appreciate any help you can provide!!!

Probably its the low ram memory . You can open your task manager and watch the ram memory usage over time. I had 8gb ram and when i was trying to build the lighting for over 1000 actors , sometimes my pc froze. I upgraded my pc ram to 16gb and it never got frozen again.

The Ram is little it is true , but for a year I never had these problems , I can not understand what happened in the last two weeks. I do not know which way to turn…

i retried, and now after just 5 second, my pc freeze again…this image is of one second before the crash…

(HWMonitor to the left, TaskManager to the right) . Disk is at 99%, but often it is 100% and the pc never freeze…