Text messages slow to send

Samsung Galaxy S4, running Android version 4.4.4_0031.  My signal strength is a steady 3 bars currently, and I having intermittent problems sending text messages.  Several times lately, when I send a text to an established contact, the message status just spins, much like the Microsoft status spinning arrow.  I have rebooted several times. I've hard booted with a battery removal, and still, it will be 3-5 minutes before some texts will send.  A minute or two later, sometimes with lower signal strength, the text message sends almost instantly.  I don't have any trouble receiving texts, that I'm aware of.

I'm using the standard Samsung text messaging client app.  It has worked flawlessly, until a couple of weeks ago.  The occurences are not isolated to a location either.  It will happen sometimes when I have full 5 bars, outdoors.

Thoughts??