Timers becoming unscheduled

We're using timers to get some loose real-time behavior in a couple of 
applications.  We've found that over the long term, our timers silently fail; 
they stop calling the function registered at timer creation.  This is happening 
both with timers that are scheduled to recur with a fixed repeat interval and 
with timers without a repeat interval--in the latter case, we are rescheduling 
the timer in the callback function itself.  In all cases, we're setting :thread 
t when calling sb-ext:make-timer, so as to have the callbacks execute in their 
own threads.

The failure we're seeing is happening when we're connecting to an SBCL instance 
using Emacs and SLIME.  We've run some tests in which we run SBCL directly from 
the shell, and we haven't seen a failure in that case.  The problem may be 
related to signal handling when SLIME/SWANK is running.

When we're just running a handful of timers that keep rescheduling themselves 
every few seconds, we can run our application for days or weeks before we see a 
failure.  When one timer fails, though, it seems to take all the rest of the 
timers in the image with it--also suggestive of a problem in signal handling. 
We've run test cases with hundreds or even thousands of timers turning over at 
once, and in those cases we can usually see a failure within minutes, or an 
hour to two hours.

Some sample code demonstrating the problem with recurring timers is available 
at http://www.deepsky.com/~tuxedo/exercise-timers.tar.gz.

Is this a known problem with the timer implementation in SBCL?  In order to be 
certain that our application won't stop executing timer-related callbacks, 
should we have a dedicated thread that sleeps and periodically wakes up to 
reschedule all the timers?  Or an external process that tickles our Lisp 
program for the same purpose?