I had the same problem with my 640 xl. After some research and lots of testing with different phones and sim cards imo the problem is in my Lumia and the 3g network. When the signal was weak or lost, my phone had hard times connecting again and receiving messages and loading pages.
When the same sim card was put in Android phone there was hic ups for some seconds, but then the programs received/sent their messages and browser pages loaded on their own. When in my 640 xl I had to manually reload pages, close open chats to be able to receive those messages.
Sim cards from other operators worked better, probably because of better network.
I almost changed my operator, because of this but testing with android fixed the problem for me.
I sold my 640 xl, bough Asus Selfie and almost no problems so far. Waited for 4 months MS to fix the problems I had with my Lumia, wished Win 10 to be a changer, but I am not a patient man and got back to Android, what a pleasant come back...