exceedingly strange behavior

I am trying to connect to an OS X 10.4.x GS4 running 1.71 from PCs.
I have to have ARD running on 5900 and have OSX set to start on 5909
From an older PC running realvnc installed a year ago I can connect on port 0 but not port 5909. On the ARD port it gives me the login screen for vnc, I login and it starts writing the remote display when it suddenly goes blank and says unknown message, but if I tell realvnc to do it again, the remote screen succeeds and I am in.
I then go to the Mac and manually bring up OS X and confirm in the log it is listening on port 5909. I also set up the java browser client for port 5909
I can then access 5909 via browser from the same machine.
Now I go to another machine (PC - recent vintage) with realvnc 4.1.1 and try the same two things. On the web browser I get: connection refused. On the client I get the same kind of behavior on the ARD port (except the screen looks like it is starting to fill in when it disappears with the unknown meesage error) Of course on port 5909 with the viewer I get nothing
I set OSXvnc to share connections (I have noticed that can be a real problem. But I am at a loss as to what is going on and why this won;t work when I have it working on so many other machines (even with ARD and OSXVNC) I have port set to 5909 on autostart with display set to auto.
The only diff in this case is the OS X machine is runnign remote profiles but I have OSX running as ladmin and the console logs show its coming up fine. Firewall is not set on the box.
I have to assume that something is killing the server. I noticed there were some comments about an issue to resolved in a future release, but I don;t know that this would explain why it would apparently work on port 5900 on one PC but not another…