I have this happen a few times a month. Not sure of cause...it might have something to do with how phone is used and how strong the connection is to sprint network. The network can be extremely slow at times which causes the internet/apps/any connection to almost freeze or time out.
I have friends who have very few apps but live on phone (calls, calendar, etc...) and they have this issue frequently. I think it could be a battery connection issue (loose connection)...these sales people are in the field and bang phones around, etc. which does not help.
Good luck on isolating the issue and resolving.