Resolution:
Yes, there is a known defect found in Tru64 that involves timers not firing correctly.  HP has a patch available for this.

TIBCO Rendezvous, as a real-time messaging system relies heavily on timers.  Without the timers, many aspects of a daemon can be affected.  Memory growth of the daemon would be possible a symptom, as messages are not being discarded after 60-second reliability.  You can request a copy of a non-RV test program called "breakcond.recursive" from TIBCO Support, which will help determine whether you are encountering the timer-related operating system defect or not.

HP has informed us that you should contact their Customer Support at HP and request the "limited distribution patch" for V5.1A.

Patch details:
=============
PTR Problem: TIBCO-877 117-2-877
Abstract: [V5.1A-H-BL3] TIBCO ISV case - Indefinite postponement of a thread;
pthread_cond_timedwait not firing properly
=============

This patch should also be included in a patch kit for Tru64 UNIX.

Are there any known issues with running RV on DEC Tru64 UNIX 5.1?