Netgear JGS524Ev2 - Slow internet speed

I have a Netgear JGS524Ev2 - 24-Port Gigabit Ethernet Smart Managed Plus Switch, which is hardwired into a Virgin Broadband router with a speed package of 500 mbps. 

The speed on network ports feeding off the switch drops to a maximum of 60-90 mbps (10/100 Mbps). Whenever I restart the switch I get the full speed, however after a few hours the speed drops. 

Any ideas on what I can change in the configuration?