iMessage intermittently not working over wifi

Hello,

Had this issue for several months, tried everything I can find online to resolve with no solution as yet.

Using an iPhone 5S, iOS 8.

iMessages over cellular network is fine. However, when on wifi iMessage is very intermittent and unreliable. This evening, my network crashed briefly and my cellular coverage took over, a message came through which was sent hours earlier and apparently still one missing before that.

Sometimes wifi is fine, sometimes as soon as I disable wifi I receive messages. This is not limited to my own wifi, it appears to be other wifi areas too.

Attempted do far:

-Network reset

-phone reset

-phone complete wipe and restore

-new router at home (when I thought it may have been that)

-removing and restarting all icloud and imessage/facetime

Has anyone had any experiences or possible solutions?

Many Thanks