thank you very much for the reply.
before a year or so, I was also suspecting VRM of the motherboard. it is very much likely that VRM should not the culprits. at least not in all cases. I want to share two main reason for that and I already shared one of the point in the main post.
1). most of the systems, which hanged like this certain way, have motherboards having at least 14+ VRM. and even just before hang occurs VRM temperature was in control everytime.
2). more is, even when there was no load and CPU utilization was around 2 to 3% .. CPU and vrm temperature was in total control ( around 40 celcius) .. even then such type of hang occured multiple time and in multiple systems.