[X4U] 2009 Mac Pro wakes, then sleeps again?!

[catsoul]

I always sleep my Macs when I'm not using them…for the past week or 2 I've noticed my 2009 Mac Pro "Nehalem" responds to being awakened, I get through the log-in and begin looking at email…

…then the screens go blank, say "no signal" and I hear the machine going back to sleep!!

Anyone know what up with this?  I wake it right back up again, then it stays awake, perhaps realizing I really want it to be awake, but why wouldn't it stay awake the first time?

Thanks!

c