Dropping idle when stopping

[russ dye]

On my 91' G20 when I come to a stop the RPMs will drop to about 250 or so
and sit there for a sec before gradually going back up to about 750.  This
has really been happening since I got the car so I gave it a basic tune up
cause the car shut off a couple times when stopping.  For the tune up I
replaced plugs with bosche plat 4+ (I know they suck, could this be causing
the problem again)  I also put in a K&N air filter, cheap fuel filter
(should I go buy a K&N one, how important are they?), cleaned the throttle
body with carb cleaner (just sprayed it in), put in mobil 1 synthetic with a
mobil 1 filter, and ran wymm's injector cleaner through the gas tank.  It
has not stalled again as it was but it has the same symptoms.  Any ideas?

Thanks for your time, Russ