A monitor going into power save mode can be due to a number off issues but it usually occurs when one connects the monitor to the PC via a display port cable. NEC has posted a notice regarding his issue here.
Note that a problem DP cable may or may not be the root cause when 'entering power save mode'. For example, my Dell U2711 often suffered from this 'power save mode' issue but when i installed two U2711's connected to my HP Z210, it would rarely suffer from this problem....
Mind you, on rare occassions i still see 'entering power save mode' with two U2711 monitors while the Z210 is running (which requires a worksattion reboot to resolve). And on rare occassions i also suffer from the monitor locking up when powering the monitor on from standby (with the Z210 off). In this later situation, i need to remove the monitor power cable and plug it back in before turning the monitor ON where it then goes through it's startup routine and is ready for the Z210 to boot up...
I doubt your PC is overheating judging by the temps you indicated (though you may want to check your graphics card temperatures). So if you are using DP cable, try DVI cable and see if your problem is resolved, else try another DP cable from another manufacturer.