Need help on Xbee transmitter randomly freezing.

I have two XB24-AWI chips in API mode (function Set: 802.15.4 / Firmware: 10ef)

I’m pressing a button attached to DI0 on the end device, it seems to just stop transmitting frames randomly after 20 or so seconds. Then will resume again after a bit of a delay.

Only relavant settings that are not default are:
AP = API Enabled [1]
DIO0 = DI [3]

Could anyone suggest possible causes for a random delay in transmitting?

Try setting T0 to a lower value.