Won't sleep

I'm having problems getting my Mac Pro Early 2008 to sleep. Sometimes it'll go to sleep (which is indicated from the PSU relay clicking and the power light pulsing), and then wake up. Other times trying to put it to sleep just hangs it on a black screen but the machine remains on, though at reduced fan speeds.

Specs.
Ubuntu 13.10 EFI Booted
Dual Intel Quad Core Xeon Harpertown
32GB RAM

Any ideas? I would REALLY REALLY appreciate this.