Environment specific performance problem with IIS7 hosted WCF svc

Question

We have several WCF (3.5 hosted in IIS7) web services that are having some strange and inconsistent performance issues. In some environments they run fairly quickly. In others they time-out even if we significantly increase the timeout. We have tried all of the advice for performance tuning an IIS 7 hosted WCF service to no avail.
The svc log however shows some odd beheavior I am not sure how to interpret. I will get a message saying Opened System.ServiceModel.InstanceContext/..... The next message will say "A message was read". all good except it takes over six minutes sometimes. I do not follow these logs well enough to understand what could be causing this.
If anybody could provide any help or guidance, it would be greatly appreciated.
Best regards,
Ryan
 Additional information: Just realized that the servers where we are having the large delays reading the message have Symantec "Data Endpoint Security" installed. Seems like it could be related. anybody have familiarity with this AV client and WCF?