problem in pasting from an X-application into emacs 21

[Howard E. Haber]

Ever since the emacs was upgraded on a sun (solaris) machine from emacs 19
to emacs 21, I've had various headaches from unexpected behavior.  The
latest involves pasting text from outside of emacs to an emacs windows (in
an X environment).

I highlight the text in Window #1, and then attempt to paste it in my emacs
window (either with C-y or the middle mouse button).  Sometimes it works and
sometimes it doesn't.  When it does not work, what gets pasted in the emacs
window is text from some previous paste within the emacs window.

Has anyone ever encountered this?  What is maddening is that when I test it
out with fresh windows, it works properly for a while.  But then it doesn't.
What is going on?  Nothing in the manual that I can see indicates why this
might be happening.

Thanks!