What's a good method for finding short duration interference?

My 440 repeater is plagued by an interfering signal that lasts a small
fraction of a second, just long enough to bring up the repeater,
and repeats randomly every 20 seconds or so.
Comes and goes...Seems to be dependent on outside temperature.
Sometimes sounds like there's hum on the signal.
First thought was that it was one of those oscillating Winegard
RV TV antennas, but it doesn't seem to drift around like they
typically do.

I hooked the repeater antenna up to the spectrum analyzer,
but the duration is too short to get thru the filters at
the bandwidth required to get any sensitivity.

I can hear it on the mobile.
I've driven around the neighborhood, but the duration is so short
that I've not been able to track it down. Strength seems to peak
at different places blocks apart...but it's hard to tell.

I have some experience with a double duckey and started designing
a doppler system, but it occurs that the short duration signal
will be completely washed out by the Switched Cap Filter. Don't see
how I'm gonna get the low rep-rate signal past the noise. Filter
types that I know about ain't gonna work.

Ideas?
Thanks, mike