portableaserve threads problems

I've been using portableaserve with Lispworks 5.1 on MacOS 10.5.8 for  
years without incident,  but three months ago switched to Lispworks  
6.0 and have now this problem,  as seen in the server log:

71.163.186.9 - - [Thu, 15 Mar 2012 17:19:14 GMT] "POST /klaggle HTTP/ 
1.1" 200 -1
71.163.186.9 - - [Thu, 15 Mar 2012 17:19:55 GMT] "POST /klaggle HTTP/ 
1.1" 200 -1
71.163.186.9 - - [Thu, 15 Mar 2012 17:20:47 GMT] "POST /klaggle HTTP/ 
1.1" 200 -1
aserve-accept-6: 03/15/12 - 13:20:56 - all threads busy, pause
aserve-accept-6: 03/15/12 - 13:20:57 - all threads busy, pause
aserve-accept-6: 03/15/12 - 13:20:58 - all threads busy, pause
aserve-accept-6: 03/15/12 - 13:20:59 - forced to create new thread
aserve-accept-6: 03/15/12 - 13:20:59 - can't even create new thread,  
quitting

The problem is happening more and more frequently,  with MTBF down to  
2 days now,  even with sparse server load.  I suspect the problem is  
that old threads do not get relinquished after connections are  
dropped.  The section of code which produces the errors is main.cl:

(let ((looped 0))
		  (loop
		    (if* (null workers)
		       then (case looped
			      (0 nil)
			      ((1 2 3) (logmess "all threads busy, pause")
				       (sleep 1))

			      (4 (logmess "forced to create new thread")
				 (make-worker-thread))

			      (5 (logmess "can't even create new thread, quitting")
				 (return-from http-accept-thread nil)))

			    (setq workers (wserver-worker-threads server))
			    (incf looped))
		    (if* (accept-thread-maybe-activate-worker-thread (car workers)  
sock)
		       then (atomic-change-wserver-free-workers server nil)
			    (return) ; satisfied
			    )
		    (pop workers)))

Has anyone any suggestions for how to prevent this problem?

Lawrence Au
Sauriel LLC