What area are you in? Sprint is adding LTE800 (Band 26) to all of their existing LTE1900 (Band 25) towers but in certain markets the installing vendor forgot to install the necessary CSFB patch. What this software does, to the best of my understanding, is it notifies your phone that it has an incoming call and switches you over from LTE to 1x to receive it. The patch also allows you to receive text messages over LTE. The Sprint version of the M8 cannot be connected to LTE and 1x at the same time due to antenna arrangement, hence no SVlte.
The next time you notice this happening go into your dialer and punch in ##DEBUG# (##33284#) and then in the screen that opens up go to LTE Engineering and see if you are connected to Band 26 or Band 25 (maybe even Band 41 but I don't think they are having the same issues with that band). If you wait long enough your phone should kick back to 3g once it realizes the tower does not have the necessary software. Sprint's current workaround for the issue while they fix it is to put the phone in CDMA only mode...... which makes me mad. That is not a workaround in my book; it is a compromise.