Any have problem with AP MR42 down in unavailable all time and can't connect to the internet more than 1 hour?
Nope, but sound like a device problem.
You should open a case at support to get an device replacement.
I really think that folks will need a bit more information than this.
What is it connected to?
What do the logs for the switch (one assumes) it is connected to show
Have you performed any cable tests/checks?
What does the local status page of the AP report?
If its just one device and the rest work I second everyone else comments.
Very it works/not works in another port or very the switch port is not having any errors, then open a case to get a replacement unit.