Virtual servers unresponsive at times

Hi- We have a pair of F5 BIG-IP i2600s running in Active/Passive failover. We have around 40 virtual servers running on our appliance. Within the past month, 2 of the Virtual Servers hit a point where they become unresponsive- we cannot ping them or connect to their respective TCP ports until we disable them and re-enable them. After doing this, all works fine for a week or so until it happens again. When we are experiencing this issue, we can connect via tcp directly to the backend servers that are members of the pools assigned to the virtual servers, so it is not an node availability issue... and I would think that even if the backend pool members were marked as 'down' by a monitor, I would still be able to ping the VIPs in question. All other VIPs are fine during this time and have never experienced anything slightly similar. I had to reactivate my F5 support account so I cannot open a troubleshooting ticket today, but does anyone have any suggestions as to what the issue could be, so I can start looking around in the meantime? Thanks.