SQL Server Bouncing Up/Down

SQL Server Bouncing Up/Down

Database: SQL Server 2000 SP3
OS: 2000 SP4

The SQL Server has been up and down five times in the last two days. We have run all hardware diagnostics against machine and it is not hardware related.

Note: Have run all DBCC's...checkdb, checkcatalog, checkalloc.... no erros in any DB.

msgs in error logs range as follows:

Log 1
- LogWriter: Operating system error 6(The handle is invalid.) encountered.
- Write error during log flush. Shutting down serverError:
- 9001, Severity: 21, State: 4.
- Error: 823, Severity: 24, State: 2.
- Database 'msdb' cannot be opened. It has been marked SUSPECT by recovery. See the SQL Server errorlog for more information.
- The log for database 'master' is not available..

Log 2 - Hours later
-SQL Server Assertion: File: <p:sqlumsincumslist.h>, line=317
Failed Assertion = 'el->m_next == 0'.
- PSS NULL for assert - raising EXCEPTION_ILLEGAL_INSTRUCTION
- SqlDumpExceptionHandler: Process 1628 generated fatal exception c000001d EXCEPTION_ILLEGAL_INSTRUCTION. SQL Server is terminating this process.
- SQL Server is aborting. Fatal exception c000001d caught.

Log 3 - 26 hrs later
Error: 17883, Severity: 1, State: 0
- The Scheduler 1 appears to be hung. SPID 0, ECID 0, UMS Context 0x03903AB0.

Last couple of hours - different times - Server froze - had to reboot.. no errors in SQL Logs...

Any suggestions: Rebuild MSDB?