I've just now upgraded the second direcTivo S2 from 6.2a to 6.4a using slicer. After slicing, & clearing the program info and todo list, I then ran tweak.sh to correct a couple of minor issues. On one, I've run bufferhack11j.tcl and updated TWP. The other hasn't gone that far yet.
Both systems are exhibiting the same symptoms. If through telnet I issue a reboot command after making some changes, they normally crash twice right as they hit 100% of acquiring satellite info, but nearly always work the third time with no intervention. I can capture a serial port log if that would help.