Are you using perhaps Zigbee groups to control multi-light fixtures ?
Note that in my experience you run into limitations very fast with multicast traffic on all EZSP based chips as well as Deconz. As soon as you start using the zigbee groups your network will become unstable. Solution is to either NOT use the multicast groups OR use a non EZSP based stick, such as the Sonoff ZBdongle-P (so NOT the dongle E !)
I’m running it with 180 zigbee devices without issues