First let make it clear that I'm not an expert, but when my PC displayed similar symptoms it was over-heating slightly. The PC stands on one one of those little trolleys so it can be easily rolled out from it's usual position under a shelf. The sides of this trolley are quite tall and they were blocking some of the cooling air intake holes in the side of the case. The PC worked fine until the weather warmed up then it started to display the symptoms you describe. I may be barking up the wrong tree entirely but it's just a thought.