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cruise contrl oscillations, uphill in '91 200tq

i've noticed some strange cruise control behavior,
with my '91 200tq wagon:

picture going up a long slope, at 2800 rpm, with a boost pressure
of about .9 and in cruise control.
i would have expected that this would feel the same as if
a driver's foot was simply on the gas pedal.
instead, i get a second and a half of very slight acceleration
followed by a second and a half of very slight deceleration
(and during the deceleration an almost inaudible sound as of
a vacuum leak or an air leak), and this recurs regularly,
on off on off on off...

my mechanic says that that is normal for these cars.
a virtually identical car behaved that way recently,
he reports, going across kansas, where there were gentle
slopes up or down for 30-40 seconds at a time.
(i'm inclined to accept that, but i can also easily imagine
that there is a characteristic problem that develops with age
in these cars. i wonder.)

so, i would really appreciate knowing others' experiences
with this kind of thing.

(gideon weisz, boulder co)