Alright guys, so I did a bit of research and below are my findings. (Not included in the charts, but on my server Mannoroth-US I believe that mushgog spawns sometime on thursday evenings. If you'll look at my times below, I believe that the rare spawns in the dire maul are on a 4-8 hour timer that starts at last spawn time or last kill time I'm not sure. Check out the data for yourself. Hope it helps. Oh and I do not believe that the spawns are there at server reset. I believe that while the servers are down the timer either stops or keeps going. Meaning that if the spawn would have happened while the server was down, it is skipped and the spawn timer resets.

***4/28/2008***
2:26:28 PM - KILL: Skarr
8:08:37 PM - SPAWN: Razza
8:09:04 PM - KILL: Razza (Griniblix says "The Razza just made more fertilizer for Mushgog!")

***4/29/2008***
1:39:00 AM - SPAWN: Skarr
1:40:00 AM - KILL: Skarr (someone else atk'd first and had tag. I killed them but couldn't loot)
///no respawn as of 6AM server shutdown///
///server goes down for maintenance not able to check for spawn immediately after server came back up///
5:43:00 PM - SPAWN: Razza
5:44:00 PM - KILL: Razza
(left maul at 11:11 PM for raid no spawn. returned at 3:15 AM)

***4/30/2008***
05:49:19 AM - SPAWN: Razza
06:36:58 AM - KILL: Razza (someone else got kill as I was afk)
01:38:00 PM - SPAWN: Razza
01:39:00 PM - I engaged got ganked
01:42:00 PM - Ganker re-engages and gets the kill.
07:27:00 PM - SPAWN: Skarr
07:28:00 PM - I engaged got ganked with skarr at 3%
07:30:00 PM - Ganker re-engages and gets the kill


(I left dire maul at 7:40PM or so and didn't return because I had raid at 11 and knew the respawn wouldn't happen till then
I went right after raid and the razza was there. There were also a lot of herbs in the arena which means mushgog spawned. I've
yet to fight him, but I want to really badly.)
***5/1/2008***
02:11:00 AM - KILL: Razza (griniblix mentions the fertilizer thing)
07:46:00 AM - SPAWN: Skarr
07:47:00 AM - Warrior engages
07:48:00 AM - Warrior gets his arse beat
07:48:00 AM - I engage and kill
02:14:00 PM - SPAWN: Skarr
02:16:00 PM - KILL: Skarr (someone else I was afk, think it was a warlock with felguard based on combat log, but maybe he had help)
08:13:00 PM - SPAWN: Razza
08:15:00 PM - KILL: Razza

Note: on the kill where the other player had the tag, I believe that somewhere in the chaos I hit skarr. Had I not hit skarr, I'm pretty sure he would have reset and I would have been able to kill again.