What did the full diag tell you?
Did the diag fail or reset.
Did the system hang on boot after diags?
Sounds like you have a bad DIMM hanging the system.
Pull half of your system boards and boot into single user mode.
If the system boots, the problem is, more than likely, on one of those pulled boards.
Add/remove boards until you can repeat the failure symptoms. Once you isolate the bad board, you can start pulling banks of memory on the board and isolate/replace the bad bank/DIMM.
or
Contact your Sun service provider.:)