Multilink PPP, 3 links work, 4 fails? Cisco 2821

[NoTx]

Hello,

We have an odd issue. We are attempting to move from a multilink PPP group of three T1s, to a group of four T1s. The current router functions properly with any three--or fewer--of the four circuits, however, the new router is unable to maintain a steady connection on any of the four circuits when all are in use; all ports intermittently come "up,up," but almost immediately return to the "reset, down." Is this, perhaps, an issue with the hardware, or is there a portion of the configuration which needs to be modified? We have at this point tried multiple 1721 routers and are currently testing with a 2821.

It may also be important to note that the other side of the connection is a 7206VXR (NPE 300).

Our config for the 2821.

Current configuration : 1791 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
service password-encryption
!
hostname GROUP
!
boot-start-marker
boot-end-marker
!
logging buffered 8192 errors
enable secret 5 $1$KRwj$BXSg297MpLufk6i1NrubM0
!
no aaa new-model
!
resource policy
!
network-clock-participate wic 0
network-clock-participate wic 1
ip subnet-zero
!
!
ip cef
!
!
no ip domain lookup
!
!
voice-card 0
no dspfarm
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
controller T1 0/0/0
framing esf
linecode b8zs
channel-group 0 timeslots 1-24
!
controller T1 0/0/1
framing esf
linecode b8zs
channel-group 0 timeslots 1-24
!
controller T1 0/1/0
framing esf
linecode b8zs
channel-group 0 timeslots 1-24
!
controller T1 0/1/1
framing esf
linecode b8zs
channel-group 0 timeslots 1-24
!
!
!
!
!
interface MultilinkGROUP
ip address XXX.XXX.XXX.XXX
ppp multilink
ppp multilink group GROUP
!
interface GigabitEthernet0/0
no ip address
shutdown
duplex auto
speed auto
!
interface GigabitEthernet0/1
ip address XXX.XXX.XXX.XXX 255.255.255.248
duplex full
speed 100
!
interface Serial0/0/0:0
no ip address
encapsulation ppp
ppp multilink
ppp multilink group GROUP
!
interface Serial0/0/1:0
no ip address
encapsulation ppp
ppp multilink
ppp multilink group GROUP
!
interface Serial0/1/0:0
no ip address
encapsulation ppp
ppp multilink
ppp multilink group GROUP
!
interface Serial0/1/1:0
no ip address
encapsulation ppp
ppp multilink
ppp multilink group GROUP
!
ip default-gateway XXX.XXX.XXX.XXX
ip classless
ip route 0.0.0.0 0.0.0.0 MultilinkGROUP
!
!
no ip http server
no ip http secure-server
!
!
!
!
!
control-plane
!
!
!
!
!
!
!
!
!
!
line con 0
line aux 0
line vty 0 4
password 7
login
!
scheduler allocate 20000 1000
!
end