I'm having similiar issues with 9 sites and it seems to stem from network latency. Version 1.60 is fine on the CPU. However drop the SP down to SP01J (Per NEC) or higher; remove any SP2 or 3. CCISoIP V1.58 is okay but if you can get your hands on it, NEC just sent me SP01X and 1.59T4 to resolve several TCP/IP issues and PVA lockups. Let me know your email address and I'll forward you the SP and PKG files.