[c-nsp] mlppp on 2801 with 12.4

[Mark Kent]

A dual-T1 customer of ours just replaced his 2501 with a
2801 running 12.4(1c).

I configured multilink ppp as usual, but I am getting some weird
performance issues and a lot of output drops on the multilink
interface.  Right now, pushing about 1.3Mbs in 1200pps, the 2801 mlppp
interfaces shows drops at about 40pps outbound from the 2801, as shown
by this line:

   Input queue: 0/75/0/0 (size/max/drops/flushes); Total output  
drops: 43858

Config on the 2801 is basically this:

ip cef
!
interface Multilink7
  ip address x.y.z.82 255.255.255.252
  no ip redirects
  ppp multilink
  ppp multilink interleave
  ppp multilink group 7

interface Serial0/2/0
  no ip address
  encapsulation ppp
  no fair-queue
  down-when-looped
  ppp multilink
  ppp multilink group 7
!
interface Serial0/3/0
  no ip address
  encapsulation ppp
  no fair-queue
  down-when-looped
  ppp multilink
  ppp multilink group 7
!
ip route 0.0.0.0 0.0.0.0 x.y.z.81

Is the above the canonical MLPPP configuration?
What causes output drops on a Multilink interface?

Thanks,
-mark