kvm-72 rhel4 (2.6.9-derived) i386 guest - rtl8139 stalling; lost interrupts?

I have observed downloads to stall on an RHEL4 i386 guest (on an x86_64 host), such that creating additional network traffic (ie. pinging the virtual host in question) will "unstick" the download. This does not happen to a RHEL5 x86_64 guest on the same host, and is quite reliably reproducible while kickstarting a new virtual machine across a network (bridged, with a different VM hosting the packages being downloaded).

To me, this smells like lost interrupts; is there anything I can do to better diagnose the issue?