11.2.0.1.0 dbconsole keeps getting unreachable status every day or so

I have 3 11g (11.2.0.1.0) databases on one windows 2008 r2 server and the dbconsoles work fine on two of them, but one of them keeps getting the unreachable status every day or so, so I can't rely on it to run backups, etc. 

where is the best place for me to debug what the problem is?  its odd to me that 2 databases run their consoles fine but one does not, so something must be different about this one stubborn dbconsole.

Any advice is appreciated.

ElonDBA