Check your Idle Stabilizer unit, located in front of the right taillight.
This unit is mounted on the inner wheelwell of the rear wheel, and if 
you have rust on this area, you get a lot of water spashing in there.

Mine was installed upside down, as well, and this allowed water to collect
inside the unit, eventually corroding the electronics and killing the poor
thing.  The wires should come out of the bottom, where they are likely to
get dowsed by water spraying in through the rust hole. (^;

This unit controls the air-bypass valve, which boosts the idle above the 
low level you are seeing.

These jobbers are expensive, too!  You can probably fix this problem by 
removing the unit and its plug, drying them thoroughly, and filling the
plug with grease when you re-assemble it.

I still need one, but without either power steering or A/C, they are not
a real critical item (they keep the engine from dying when you steer to
the stop, or the A/C kicks in -- without either, you can simply up the
idle speed a tad to compensate for a while, but probably won't pass smog).

malcolm