bonding driver dropping packets

Trying to debug a problem with an application I own that should be
unrelated to bonding but with my appl installed, I see a 5-12% packet
loss when pinging the bonded IP. I'm not pushing the system, pings are
the only thing going on besides my appl running in the background.
Kernel version is 2.6.18-92.el5 on RHEL5.2.
Using active-backup config:
Ethernet Channel Bonding Driver: v3.2.4 (January 28, 2008)

Bonding Mode: fault-tolerance (active-backup)
Primary Slave: None
Currently Active Slave: eth0
MII Status: up
MII Polling Interval (ms): 100
Up Delay (ms): 0
Down Delay (ms): 0

Slave Interface: eth0
MII Status: up
Link Failure Count: 0
Permanent HW addr: 00:09:6b:b5:fd:ce

Slave Interface: eth1
MII Status: up
Link Failure Count: 0
Permanent HW addr: 00:09:6b:b5:fd:cf

If I change the interfaces to not be bonded, there is always 0% packet
loss when pinging either or both interfaces.

Using tcpdump on bond0, I can see that some packets don't appear to be
received -- which doesn't make sense since I don't have this problem
when the interfaces are not bonded. Using tcpdump on the slave
interfaces, it doesn't show the received packets, only the transmits
over eth0 (since it's primary).

Any pointers as to what to look at to debug?