Sounds like the service pack installer is having problems connecting to the instance via shared memory. The service pack attempts to connect via shared memory and places the sql server in single user mode, after it checks for connectivity problems (i.e. osql.exe -Slpc:InstanceName, where InstanceName is your SQL Server instance name). If it cannot connect via shared memory because of a Windows resource problem, a reboot is in order.

At what point in your sqlsp.log are you noticing the issues? Immediately, or after the initial checks have been performed? If it is after the the initial connectivity check, there may be something else going on that we have experienced in the past, but I would rather not type our problem/resolution if it is not relevant to what you are experiencing.