Do you have a view with "Polling Engine Status"? If so, what is the "Last Database Sync" value for each of your polling engines? Just want to confirm that not only are your pollers still running but they are also still syncing to your database.
I see this from time to time. It would get weird when an interface I know is gigabit was showing 200% utilization but only pushing about 200Mbits. What you may be seeing is a problem that, I believe, has little to do with NPM. Are you running Cisco gear? Sometimes, Cisco switch ports that are set to autonegotiate will renegotiate speeds for some unknown reason. When this occurs, depending on your polling cycle, those interfaces may incorrectly show at 10.0 Mbps Rx/Tx until your next polling cycle.
I see this happen most often when the switch ports in question have recently bounced or if many ports on the switch in question have recently bounced.