Too many node-internal operations are scheduled but not processed due to CPU or disk I/O problems.
If the node-internal log is out of space, the history files contain messages showing HIGH LOAD on the node internal log.
Check CPU usage, as described in Improving CPU Utilization
If CPU utilization is not a problem, check the disk I/O. If the disk shows contention, avoid page faults when log records are being processed by increasing the data buffer size with hadbm set DataBufferPoolSize=... If there is disk contention, follow the solutions suggested in Is There Disk Contention?