Roaming VLANs 991 - 994 are used with unbridged SSIDs to help route traffic back to the originating access point the client roamed from. If an SSID is bridged, then the roaming VLAN is not used, client traffic is always bridged directly onto the local network. Your switch must be properly configured to use roaming VLANs.
In some cases the firmware components used to route traffic over the roaming VLANs can lead to high processor load. Additionally some devices, such as modem/router combination devices have compatibility problems when the Roaming VLANs are enabled. In both of these cases, the Roaming VLANs should be turned.
The Roaming VLANs can be turned off by going to Configure -> Advanced -> Roaming VLANs.