Protocol Driver Error - users getting dioconnected from applications after connecting successfully, in the Citrix XenApp / Presentation Server forum on BrianMadden.com

Our biggest problem, that I've tried to troubleshoot from several angles, is users getting disconnected from their Citrix session. They'll be working away for an hour or so and then get a Protocol Driver Error popup and the session disconnects. Its not a network issues as we have SNMP configured on all routers to alert us if there are any outages. We have 2 CSG's using NLB and if one user gets diosconnected, other users can continue to work. We currently have 6 production farm servers (which we'll be increasing to 9) which are PS4.0 and pacthed up to date. The disconnects are not linked to a particular server either, though they are all equally loaded. Clients are Web ICA 9.23 on Windows XP.

In short, it is extremely difficult to troubleshoot as it is so intermittent. Also, most of the results from internet searches (this site adn the official Citrix support site) mention Protocol Driver Error relating to problems launching an application (usually STA errors) rather than after an application has been successfully connected?

What can be causing these disconnects? Would farm server load cause it? CSG load?

Thanks