sounds like the Idle Valve Speed Controller this is exactly what my astra gsi did and the previous owner said hid bought a new and it looked like he had when i took it off so i cleaned it with carb cleaner which usually fixes them but there was no difference so i went to the scrappy and bought one after exhausting every other possibility for solving my idle problem it cost £20.
now idles perfectly no probs i guess the one in mine was just knackered the seam very tempremental and you can't tell if there damaged just by looking at um.
my advise get one of a 20L 16v then you know you have the right one!!!
another thing buy a can of carb/fuel injection cleaner and spray everything under your sfi box that looks dirty and clean the pipe from the rocker cover to the inlet this helps aswell as this normally cloggs up with dirty oil.
Mine used to not idle when warm, until I took it for a RR tune. I believe the guy adjusted the pot on the AMM, which in general is not encouraged to be done at home. I'm not even sure exactly what it varies (AMM signal I assume).
Cleaned the icv,
Checked for air leaks, none that i can find anyway
Plugs are gapped fine,
Cleaned the pipes,
Cleaned the throtle body (was filthey but clean now)
It will now idle when warm but the revs are up and down like a yoyo.
Cam timing looks ok all the timing marks still line up.
The exhaust is blowing where the downpipe joins to the system would this have any affect on the idleing problem???