ISA 2006 Firewall Service Stops every few hours

A Windows 2003 Server SP1 with ISA 2006 installed was working fine for several months, then suddenly it started stopping every few hours, sometimes after few minutes. The system has Websense ISA plug-in and Symantec AV client 10.1.2
The errors in the Event Log are:
System Log:
Event ID 7034 The Microsoft Firewall service terminated unexpectedly.  It has done this (x) time(s).

Application Log:
Event ID 14057:
The Firewall service stopped because an application filter module C:\Program Files\Microsoft ISA Server\w3filter.dll generated an exception code C0000005 in address 6475D3E5 when function CompleteAsyncConnect was called. To resolve this error, remove recently installed application filters and restart the service.

Event ID 1000:
Faulting application wspsrv.exe, version 5.0.5720.100, stamp 44a3a98a, faulting module w3filter.dll, version 5.0.5720.100, stamp 44a3a962, debug? 0, fault address 0x0006d3e5.

Event ID 14079:
Due to an unexpected error, the service fwsrv stopped responding to all requests. Stop the service or the corresponding process if it does not respond, and then start it again. Check for related error messages.
When I start the service:

Event ID 14176:
Disk cache F:\urlcache\Dir1.cdat failed to initialize. Some errors were encountered when ISA Server restored specific data cache files. ISA Server will now attempt to recover these files. These errors may have occurred because there was not enough time to complete all necessary shutdown operations, when ISA Server was previously shut down. To avoid this in future, you can increase the value of the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\WaitToKillServiceTimeout registry key.  Identify the reason for cache failure by examining previous recorded events, or the error code. Check that the disk is connected and that it is not corrupt.  (Internal code: 503.7361.5.0.5720.100)

Followed by:
Data cache file F:\urlcache\Dir1.cdat was recovered. If the operation did not complete successfully you may use the error code in the Data area, which indicates the cause of the failure. Recovery operation result is: The operation completed successfully.

I have been struggling around this for 3 days now.
Please help.