It sounds like you have a bad radio module or perhaps it is heat sensitive such that when it gets warm, it stops working.
It's also possible that the power cube / injector is defective (not enough power, loses power when it warms up).
It might also be as simple as re-seating the radio module in the AP .... sometimes oxidization can accumulate on the contacts of the module (re-seating the module would help this too).
Unfortunately, the screws to get into the AP are "posted" Torx (a hollow socket with a pin, so a regular Torx driver can't be inserted).
The radio update modules come with the necessary tool, and they're also available in some "security" bit sets. I think it's a T-8 or T-10 size.
If you have an idea of how long it runs under normal conditions (check the log of power-up time versus when the clients start to drop), try putting the AP in the 'fridge to cool it off, then power it up and see how long it takes before the clients start dropping again.
Make sure it has adequate ventilation and air circulation around the AP so it doesn't overheat.
Give it a shot and let us know.
Good Luck
Scott
