I have had similar problems over slow telephone lines also. I believe that the problem lies with in the telephone system itself. Loss of packets when trying to connect to Citrix via the internet would cause this problem.
For instance, when I am testing in the office through a telephone line and ping our servers - some packets will come back as "destination unreachable" while others will come back successfully. This is the phone system - not citrix!
I would try connecting through one of the phone lines and try pinging your server - I bet that you will see that packets are being dropped and timing out. This will tell you that it is the actual connection - not citrix or your servers.
Hope this helps!
Pinkman MCSE, CCA