Welcome! It doesn't sound like interference, rather how your QoS is setup on the devices. Do you have an application or network appliance that is controlling the bandwidth to your customers? Or are you limiting them at the radio level (using QoS)?
If you are using the default settings (and letting the radio manage bandwidth), it is possible that one or more devices is going well beyond the 1Mbps upload speed. You would want to set your sustained downlink/uplink speeds to a data cap for the plan (i.e. 3000kbps / 1000kbps) and either consider using bursting (see attachment) or configure the burst downlink/uplink speeds the same as the sustained. Otherwise, the data cap per customer is the entire bandwidth of the sector.
Here are some additional links to get started with PMP450 (in addition to the training at https://learning.cambiumnetworks.com/ ):
http://community.cambiumnetworks.com/t5/PMP-FAQ/Throughput-vs-Range-An-Explanation/m-p/37199#M269
- White Paper PMP450 MaxBurst.pdf (607 KB)