My iPhone 4S screen wont come on after being locked.

Sporadically when I lock the phone then try to use it later the screen will not come on. I try pressing every button combination I can think of and the phone will not respond. If I do a reset (home and lock button hold) the phone will restart and work correctly. When this happen I always end up missing texts or calls because it is impossible to tell when the phone became unresponsive.

I would like to know if this is a software problem and I should just wait out the inconvenience, or if this is a hardware problem and I should get it fixed.