Intermittent voice channel dropouts on conference calls

We have conference calling set up and for the most part it works fine. On some conference calls, however, 5 or 10 minutes into the call with 3 or more callers the voice channel for one of the callers will drop out for 15 seconds or so. All participants can hear at all times, however, one individual will be unable to be heard by the other participants. A few minute later, another individual’s voice channel will drop out for 15 seconds or so. There is really no pattern to who will drop out or when it will happen.

It is almost like they were muted for 15 seconds. We have a ticket open at our DID supplier as well. The DID supplier has indicated they proactively put this DID onto a server that would help with codec negotiation in order to see if that helps.

The details of our system configuration are:
P4 3GHz 4GB memory, Asterisk 1.4.11, no Zaptel hardware.

meetme.conf

[general]
audiobuffers=32 ; The number of 20ms audio buffers to be used
; when feeding audio frames from non-Zap channels
; into the conference; larger numbers will allow
; for the conference to ‘de-jitter’ audio that arrives
; at different timing than the conference’s timing
; source, but can also allow for latency in hearing
; the audio from the speaker. Minimum value is 2,
; maximum value is 32.
;
[rooms]
;
; Usage is conf => confno[,pin][,adminpin]
;
; Note that once a participant has called the conference, a change to the pin
; number done in this file will not take effect until there are no more users
; in the conference and it goes away. When it is created again, it will have
; the new pin number.
;
conf => 1234
conf => 1235,678
conf => 1236,679

;Company A Conference room
conf => 1237,680

;Company B Conference room
conf => 1238,681

;Company C Conference room
conf => 1239,682

;Company D Conference room
conf => 1240,683

sip.conf

;conference call DID

[IP Address]
host = IP Address
type = friend
insecure = very
context = CONTEXT
canreinvite=no
disallow=all
allow=g729
allow=ulaw
allow=alaw

extensions.conf

[CONTEXT]

; Conference Rooms

exten => 1888xxxxxxx,1,Wait(1)
exten => 1888xxxxxxx,2,Background(conf-getconfno)
exten => 1888xxxxxxx,3,Read(CONFNUM)
exten => 1888xxxxxxx,4,Set(CDR(accountcode)= ${CONFNUM} Conferencing)
exten => 1888xxxxxxx,5,MeetMe(${CONFNUM},Ms)

Any ideas on what may be wrong in our setup or pointers to how this problem may have been addressed by others is appreciated

Thanks,

Mike