IPhone 4s won't send/receive some messages?

Hiya,
Basically my iPhone 4s won't send or receive some messages from one particular contact. It will all be fine and then after a while my friend just won't get my messages, or I won't get his, and it's not down too bad signal.

I've looked on loads of forums to try and find a solution but so far they haven't been very helpful.

Has anyone else had a similar problem and managed to fix it? Or is my phone on the way out?

Many thanks
By the way, my friend does not have an iPhone and I'm with orange.

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