Latency in system at same time every day

Our client is experiencing gradual latency almost every day at almost the same time (around 3:00PM). By "latency" is meant poor response time from different, disparate parts of the system: the browser application as well as operations using the putty terminal emulator. Occasionally the latency culminates in the putty (terminal emulation) sessions closing, leading us to wonder if there isn't a network issue that needs looking into.