iphone 6+ message lagging and crash.

Im using a iphone 6+ with ios 8.1, my message is always lagging and crash. It lagging 30-40 times a day, sometimes all my message are gone and i have to restart my iphone to get back my message. Im wondering is there any way to fix this problem. I have tried to backup my phone and do a factory reset with itunes, but it didn't help the problem. My message was saved 1.2gb memories, i have tired to deleted some of my old message, but it it automatically restore the message(so i don't know how to clear my old message). Im wondering if anyone can give me any idea to fix this problem.

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