iMessage not properly sending/receiving messages

I have been using iMessage for years, without any problems. However, a few weeks ago iMessage started to behave weird. Sometimes everything works fine, and sometimes it takes ages to send and receive messages. I cannot send or receive any messages for a few minutes, and then suddenly I get ten messages at once, sometimes even in the wrong order. This has never been an issue for years, it only started two weeks ago. And yesterday, a friend sent me a few messages that did not arrive at all!

How can I solve this?