idle surge is quite common.
a) clean the throttle body. This is best done by taking it off.
b) check for cracked vacuum hoses (the set on the throttle body)
c) check the intake hose for cracks.
d) check the vacuum hose to the brake booster
e) R/R the harness connections from the throttle body to the MAF sensor in
the air cleaner, using some WD40. Check the connector pins for corrosion.
You can usually clean these with a v.small screwdriver and more WD40.
f) finally, adjust the idle speed up with the idle air control valve. This
is under a 10mm black cap on the top of the throttle body.

Not immediately related to surge, but get your emissions tested. Quite
likely the cat and / or the O2 sensor are no longer functioning by now.