|A rebuild during the reboot is normal and expected in these situations.|
As for the dump file: If the file: SYSDUMP.DMP does not exist in SYS$SYSTEM, then the pagefile can be used. The pagefile must be large enough and the sysgen parameter SAVEDUMP should be set to 1 which will preserve the dump until it has been analyzed within the pagefile. If you save the dump within the pagefile, it is recommended that you "COPY" it to another location and release the pagefile pages.
To do this:
SDA> COPY SAVE_location
Where SAVE_location is a disk directory large enough to contain a dump file. This command will release the pages back to the pagefile.
RE: The crash itself.
Without an understanding of the application that is crashing the system, it will be difficult to help much further. Get a valid dump, save it from the pagefile and then provide the output from a CLUE CRASH. Hopefully that will provide enough info to go to the next step.