intermittent bsdtar/jemalloc failures

While building a local release bundle, I sometimes get bsdtar failing 
(and dumping core) as follows below. Worse, as can be seen below, it 
doesn't stop the build unless I happen to notice and it yields an 
incomplete package.

a usr/src/sys/netgraph/ng_checksum.h
a usr/src/sys/netgraph/ng_message.h
a usr/src/sys/netgraph/ng_echo.c
a usr/src/sys/netgraph/ng_gif.h
<jemalloc>: jemalloc_arena.c:747: Failed assertion: 
"nstime_compare(&decay->epoch, &time) <= 0"
Abort trap (core dumped)
sh /usr/src/release/scripts/make-manifest.sh *.txz > MANIFEST

What causes this? Build machine is a 2x4-core Intel box with ZFS 
file-systems all around. I tried stopping NTPD temporarily but the 
failures persist .. sometimes :-(

I've seen this at different points in the archiving process so it 
doesn't seem specific to building kernel.txz.

Any thoughts?

	imb