Have you verified that the resolution is supported by the monitor? What's the native resolution of the monitor? And where did you get that modeline? Are you sure it's correct?
Now, I don't know anything about a SUN machine, but is this verbatim, from the file?:
Horizontal Sync 30.0-81.0
Vertical Refresh 56.0-75.0
I ask because I've never seen it so different from:
but then, like I said, I haven't a clue what this OS or machine are like, or what version of X it's running.
PS - I have two Samsung WM2243's going, with no such issues.