4.8-R: strange hitch in user ppp Rx data flow

[Carl Mascott]

FreeBSD 4.8-R
user ppp
P II - 400
128 MB RAM
56K ext. modem, 45.3K connection

When I updated from 4.3-R to 4.8-R recently I immediately noticed
a strange hitch in the flow of received data from user ppp to
TCP/IP applications.  For example, with the BSD ftp client, when
receiving a file, the progress always stops around 113 KB and
ftp reports "stalled" for about 10 seconds.  Then the ftp
client suddenly shows 170 KB received and for the rest of the
transfer there are no more hitches.  This is 100% repeatable.
During the 10 seconds when ftp's progress is "stalled" at 113 KB
the RxD LED on my external modem shows continuous data flow.

Both Netscape and Opera show the same hitch under 4.8-R.

I tried transferring a large (40 MB) file across my 10 Mb/s LAN
several times with the ftp client and never saw any pauses.

4.3-R definitely did not do this.  Anybody got any ideas?
I've already tried the 4.3-R ppp binary on 4.8-R: same
behavior.