Stabilizing serial port interrupt latency?

[AGC <agcarver+netbsd%acarver.net@localhost>]

Are there any tweaks available, whether in user space or the kernel,
that might help stabilize serial interrupts on an IPX?  I know it's an
old, slow machine but the latency itself isn't an issue, it's the
stability of the latency.  I'm still trying to use it as a time keeper
so I have the 1 PPS signal from a GPS receiver plugged into the DCD pin
on the serial port then ldattach that to provide PPS.  However, when
ntpd is running, I see the PPS jitter is unusually high (in the ms
range).  The same GPS is feeding another machine running Debian and
there the jitter is less than 5 us.  My oscilloscope says something
similar, too.  Given two out of three devices agree that the PPS signal
is stable I'm attributing the jitter seen to interrupt handling on the IPX.

It's running headless, only sshd and ntpd are running so it's not
swamped with processes.