Citrix sessions freezing for Remote Users in Presentation Server 4.0

We have a Citrix Presentation Server 4 environment on Windows 2003 Standard SP2. There are 2 servers in the farm.  The IMA database is on SQL 2000.  The servers are HP ML360 G4's with dual proc's and 3 GB of ram.
We are running the following applications; Office XP Professional with SP3 and MS Navision 4.0 with multiple customizations installed that prevent us from easily going to SP3.  IN fact the Manager of the department responsible refuses to implement SP3.
Our Network is set up as follows
Head Office (Citrix farm)  10 Mb Wifi Internet  (40 Users local)  Fortigate 200a
Alberta South 10 Mb Wifi Internet on same backbone as above (10-15 remote users) Fortigate 100a
Alberta North 5 Mb Wifi Internet (5 remote users) Fortigate 60
Manitoba rural DSL (5 remote users) Fortigate 60

Our switches are all HP Procurve switches.

For users in all of our offices we use Citrix Program Neighborhood Agent, and any road warriors use Web Interface.
The users in the head office have no issues other than normal Citrix printer issues.  The users in my remote offices all experience numerous slow downs and freezes thruout the day of up to a minute at a time.  They are seldom disconnected. This seriously impacts their work performance.  These issues are compounded by intermittent Microsoft Navision errors when users try to print which restarts the application.  

Session Reliability has been disabled and the ICA Keep Alives are set at 15 seconds.

They have been complaining to the COO and CFO, neither of whom are tech savvy (to say the least).  
Our domain is a Windows 2003 Native domain with a DC in each remote office which also functions as Global Catalog, File and Print Servers.

Does anyone have any ideas on how to resolve the issues of sessions freezing or what may be causing them?

Thanks in Advance!