My calibra does exactly the same thing, it did improve by resetting the ecu but seems to be slowly creeping back. If I blip the accelerator it drops to around 800rpm so I think I've got a sticky throttle somewhere.
I had the very same problem on my Corsa Gsi afer fitting a new head.
The car went back too vauxhall 4 times they kept on saying that it was an air leak on the inlet manifold .Each time i got the car back and put a new throttle body gasket on (which is no an easy job with the head still on the car.
This did not rectify the problem i then changed the plugs.ht leads still the problem occured.
One day i was driving paseed a vauxhall dealership and the problem started too occur so idrove in and asked them too look at it.
It was put on tech machine whilst the engine was running and they found out that the throttle position sensor plug was faulty and was shorting out but was not regestring a fault.
They just resoldered the plug to fix it.
A bit long winded reply,but i haope this helps.
just to complete this... in case any of you experience similar probs..
got the fault codes read.. it had registered crank sensor, cam sensor, and idle air valve.
fitted new crank and cam sensors, cleaned out idle air valve and connections
also replaced all engine breather hoses (ones that get carboned up) and fitted the breather mod that comes as standard on the vectras..
Cleaned the breather stub bit that goes into the plenum chamber (has coolant pipes going through it) - it was blocked solid, and while i was at it cleaned the whole throttle assembly..