I had a Vx (i just lost it, but I'm getting a IIIc this week), and I found that to get it to work, I had to have the latest of everything..
the lastest PalmOS on my Plam
the lastest USB updates on my Mac (g4 sawtooth)
the latest Mac OS - 9.0.4
and the latest Palm desktop software - 2.6.1
Also - i have found that these problems can often be attributed to USB device probelms - specifically, hubs. If you can, try to sync with AvantGo the very first thing when you boot up - and have your cradle plugged directly into the Mac. You are doing this to debug your problem and take out as many variables as possible. I found a bad USB hub this way myself.
You must also have your channels setup at AvantGo. Try only one or two channels for now - so as to debug if i is a problem with the speed of your connection. I know that even with a cable modem, a full compliment of channels can take me 5 minutes to sync. I can't imagine how slow it is with a 56k internet connection.
In any case - try to take as many factors out of the situation to see if its a problem with your Palm, cradle, or something else basic.
I would even suggest create a boot Zip or other disk - install 9.0.4 clean, install Palm Desktop 2.6.1 clean, install AvantGo clean - and plug directly into the machine.
Let me know how it goes!
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