Can Cisco 2621XM handle 4 T1's using MLPPP?

I work in test on a product that supports MLPPP with 4 T1's.  I have configured a Cisco2621XM for MLPPPx4 using 2 VWIC dual-T1 controllers.  When performing an FTP download, the bandwidth drops to zero every few seconds, resulting in a choppy graph of bandwidth over time, which peaks at about 5Mbps, but average throughput is significantly less.

If I use two of my company's product back-to-back, the throughput graph is quite steady at around 5Mbps (would like to see more, but at least it's steady).  If I place two Cisco2621XM's back-to-back, I get pretty bad results, though different (whole T1 lines appear to drop out for long periods of time).  I'm beginning to believe I need to invest in a higher-performance router.  

During the data transfer on the Cisco2621XM, "show processes cpu" says about 30-40% cpu is being used, for what it's worth.

Here's my running config:

Router7008#show run
Building configuration...

Current configuration : 1752 bytes
!
version 12.3
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Router7008
!
boot-start-marker
boot-end-marker
!
!
no network-clock-participate slot 1
no network-clock-participate wic 0
no aaa new-model
ip subnet-zero
ip cef
!
!
!
!
!
!
!
!
!
!
!
!
!
!
username ******** password 0 ********
!
!
controller T1 0/0
 framing esf
 fdl both
 clock source internal
 linecode b8zs
 cablelength short 133
 channel-group 0 timeslots 1-24 speed 64
!
controller T1 0/1
 framing esf
 fdl both
 clock source internal
 linecode b8zs
 cablelength short 133
 channel-group 0 timeslots 1-24 speed 64
!
controller T1 0/2
 framing esf
 fdl both
 clock source internal
 linecode b8zs
 cablelength short 133
 channel-group 0 timeslots 1-24 speed 64
!
controller T1 0/3
 framing esf
 fdl both
 clock source internal
 linecode b8zs
 cablelength short 133
 channel-group 0 timeslots 1-24 speed 64
!
!
!
interface Multilink1
 ip address 10.10.45.1 255.255.255.0
 ppp multilink
 ppp multilink group 1
!
interface FastEthernet0/0
 no ip address
 shutdown
 duplex auto
 speed auto
!
interface Serial0/0:0
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1
!
interface FastEthernet0/1
 ip address 10.10.20.103 255.255.255.0
 duplex auto
 speed auto
 no cdp enable
!
interface Serial0/1:0
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1
!
interface Serial0/2:0
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1
!
interface Serial0/3:0
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1
!
ip http server
ip classless
ip route 0.0.0.0 0.0.0.0 10.10.20.10
!
!
!
!
!
!
!
!
!
line con 0
line aux 0
line vty 0 4
 login
!
!
end