All 4 SBS services, should be in running state and should not stop after certain interval.
IP Address might be missing from in initialPXEConfigPath.txt.
Location of file -C:\Program Files\Altiris\Altiris Agent\Agents\Deployment\Task Handler\SBS\initialPXEConfigPath.txt and ensure the following two lines are present, with correct IP address:
Server_Listeners="10.3.0.1" (or whatever your site server's IP is)
Mtftp_Listeners="10.3.0.1" (site server's IP)
If this information was missing, or even if it was correct, reset this registry key to 0:
HKEY_LOCAL_MACHINE\SOFTWARE\Altiris\Deployment\Symantec Boot Services\HasSbsRegistered
And restart all the 4 services.