I have Ubuntu 12.04 running on my laptop as well as desktop. There are no issues on my laptop, although it does hang momentarily when the load increases but becomes responsive soon enough. However my desktop hangs and never becomes responsive again. The only way is to force restart it. I've observed it happen mostly when I'm streaming videos. It may freeze even when I use other load heavy programs like GIMP. When I was running 10.04 on Desktop, it often used to restart automatically when I ran such heavy programs for quite sometime and I thought the system is overheating and I need to install an extra cooling fan but now after upgrading to 12.04 it only hangs and that too just after starting the system for only a few minutes. This, to me, indicates that the issue is not with heat dissipation.

My Desktop config is:
AMD Athlon 64 2 cores,
3gb in 2DIMM ddr2, 1 chip is 1GB Ram and other is 2GB RAM. However both these chips have different frequencies.

I've tried the following two steps

1. /usr/lib/nux/unity_support_test -p
All the parameters indicate 'Yes' here.

2. Use bleachbit

but neither resolve the issue. Could someone please advice me how detect the issue and resolve it. It is really annoying when the system hangs so frequently and I'm having to avoid necessary but load heavy programs for this reason.

Thanks in advance.