Printers just do what they're told. If they do produce incorrect print it is usually in the form of gobbledigook (incomprehensible print). They are not sophisticated enough to change to italics, (in the same font?), for a short while and then go back to normal again.
Does it only happen when you are using the one program, or several different ones on your computer?
If it is only one program where it occurs from maybe you should check the program (is it Word or some other Office type program etc) and see that when it does happen that there isn't a format change (even hidden) in the particular line(s) in the text.
Does it only happen with a particular font?
If so maybe the font has become corrupted.