Answer
The default number of errors before trying another codebase is 20, configurable with the Agent registry key:
HKLM\SOFTARE\Altiris\Communications\MaxErrorsPerServer
You could try reducing this number to force Agents to find a new codebase sooner, but will cause more network traffic
The NS gives out 10 codebases at a time to Agents for SpeedTesting when Sites and Subnets are not configured. Configurable in coresettings.config:
<customSetting key="MaximumCodebasesPerPackageServerPackage" type="local" value="10" />
The Agent then randomly chooses 6 of these codebases and speedtests to pick the fastest server, this server then remains the primary codebase for 6 hours until the speed is checked again.
Configured in the Agents registry:
HKLM\SOFTWARE\Altiris\Communications\MaxServersToCheck (default 6)
HKLM\SOFTWARE\Altiris\Communications\Speed Expiry (mins) (default 360)
If the MaxErrorPerServer threshold has been reached then it will choose the next fastest Package Server from the Speed Test to download its packages.