Android sleep of death (SOD)
I would seriously consider rooting and installing a custom ROM.  I've installed custom ROMs which gave me the SOD issue - my point is that I believe SOD is in fact tied to what ROM (whether stock or custom) you are running. This question details various things on rooting.  For specific directions on rooting and custom ROM for your phone model, you'll have to do a search here for that or on Google.