Hi,
First of all, all tires properly balanced and inflated? Suspension
system in good order? I drove Subaru for many years. Even now there
are two in the family(kids'). I may be insensitive person but did not
experience such split second phenomena. But then Our cars have upgraded
suspension and tires.

I noticed a somewhat similar sensation when I first drove
an AWD car- a new Subaru Impreza that I tested and later
bought.

I took the car on the highway and got it to about 85 mph
for a moment, I could see the car salesman sitting next to
me not very happy about this speed, but he didn't say
anything.  Probably was aprehensive that I may take it even higher.

I remember noticing that the car very slightly jerks,
at first I thought it was gusts of wind blowing from the
side, on an elevated freeway.  Soon it became apparent
to me that it's the AWD engaging/or compensating somehow
over potholes and bumps.

I bought the car next day. I have about 60k miles on it now,
and don't even notice this sensation anymore.  Maybe
because I just don't drive fast on bad inner city roads.

M.J.
2000 Impreza L Coupe

Have the alignment checked.

I experienced this in mine a few years ago.  It had been pulling to the
left when I dropped it off at the dealer for a service.  They didn't
have proper equipment to fix the alignment, but they "adjusted the toe"
and sent me on my way.  As I drove home I noticed exactly what you are
describing.  If I hit a bump or a pothole on one side, the car lurched
in the other direction, to the extent that it was barely under control.

I took it to an actual alignment shop the next morning, and when they
finished the problem was no more.

Good Luck,

Dave Riesz