In many causes the owner THOUGHT the machine was going to sleep because the display ANNOUNCED "going to sleep".
This is not what was happening at all. The PC did indeed turn off or fail in some way but the PC was not in sleep mode. The monitor is in my opinion TOO SMART and misleading folk about the problem.
Try this. CLEAN YOUR DESKTOP like you see at http://reviews.cnet.com/4520-11319_7-6240575-1.html
I have Hp pavilion and when i will work or continuously use the system for more than 3 hours it will automatically sleep. I will check the settings all the settings are ok my system is in never sleep mode what to do.