Serious problem with Thunderbird on Athlon X2 3800 Windows XP x64

Ever since upgrading to from an Athlon 64 3000 to my new Athlon X2 3800, 
Thunderbird has been behaving poorly.

The very first few times it checks mail after a reboot it's fine, but it 
progressively starts to develop a problem where every mail check results 
in an immediate network error.

Thunderbird is the only application on my system exhibiting this 
problem.  I was using 1.0.4 when it started.  I've since upgraded to 1.5 
RC1 and it persists.

My system:

Athlon X2 3800 Dual Core Processor at 2.01 GHz
4 Gigs of Corsair Value RAM DDR 400
MSI Neo4 Platinum SLI
MSI GeForce 6600GT

I'm using Windows XP Professional x64.  I have installed the AMD CPU 
driver for the Athlon X2 3800.

I'm willing to download the sources and attach a debugger to the program 
if someone can lead me to clear instructions on setting up the environment.