Idle issue

Hi, I have a 99 forester gt auto that has an erratic idle some of the time. It idles between the 1st and 2nd marker on the tacho and then will drop below the 1st marker and feel like it will stall. When it is idling rough it lacks power and surges a bit, also heaps of black smoke comes out the exhaust. Other times it is running as it should. any Ideas??

First thing I'd check is the IACV.

Make sure its clean - give it a good squirt of UEC.... and make sure you do an UEC regularly.

Yep UEC works wonders....
Might need two cans though if it has not been done for a while.
I use a product, Air Intake And Upper Engine Cleaner, made by a Queensland based chemical company, Austech Chemicals 07 32048511, which, they claim, is the same as SA459, and I have found to be very good at a fraction of the price of SA459.
I use a can each service interval.

And use it BEFORE you change the oil/filter ....