why does my Mac Pro keep cycling on and off while in sleep mode?

I have almost same phenomenon while sleeping mode. In my case, Mac Pro wake on and off when trun on and offf other electric equipments in my room.

If you have "Wake for network access" checked in the Energy Saver preference than your Mac may periodically (about once an hour) to update it's network settings as described here:

http://support.apple.com/kb/HT3774