Ok, found similar article on ISAServer.org, thanks J.F, good reading:
1) Make sure caching is enabled (it's not enabled by default).
It is by default in later Small Business Server 2003 Premium R2 releases of ISA 2004 (July 2007), the HTTP TTL is set to default max = 1 day, should one increase this? Where do you compromise on storgae vs cache held? Remember this is an SME, therefore the client uses the Server for Everything (budget) e.g. Authentication, Comms, Edge Firewall, DC, DHCP, DNS, VPN/RRAS, IIS (for OWA), Exchange, FileServer, AD, etc.
2) Make sure DNS is configured correctly on the ISA Server.
Yes, the clients Server has 2x Network interface cards - the external is a static IP, given a static Public IP pool from ISP. And once again, in the Later release of SBS 2003 R2, the DNS is configured correctly by default along with the actuall NIC's(i.e. run "Continue Setup Wizard" + "Configure E-Mail and Internet Connection Wizard", thus external NIC points to internal DNS, internal DNS is set to listen only on internal IP and 'Forwarders' are set to the external DNS Static IP's given by ISP along with static IP address pool range)
3) Make sure your DNS infrastructure is running healthily.
No config errors in event log, all seems healthy.
4) Try to have all users' computers configured as Web Proxy Clients.
Have not even set ANY pc's up to the domain as of yet, still only running the Server in this environment, but facing sporadic connectivity errors, and per usual the ISP/comms people say it is not them. (Mind you I am in SA, copper cable could be stollen in some or another area again)
5) See KB905179 and KB837572.
Very helpfull along with the IsaBPA tool - have modified reg, still no real effect.
Any other tips welcome, else I will need to go back to Comms guys, but would rather have confirmed nothing wrong at my end (i.e. the server is setup 100% and should not slow things down due to ISA 2004 proxy servervice)