If it stalls when you dip the clutch (i.e. coming up to a junction and slowing down) then it is probably your Throttle Body You need to clean it approk every 20-30k, I had to do this on my 1.8i 16V 1999 Astra also. I think it is time I did it again soon as mine is starting to drop the revs low when I dip the clutch
Might be worth checking the Throttle Body though as Vauxhall don't clean them (even in a major service )
Just follow the air intake pipe from the air box to the top (just above the TB), undo the jubilie clip, pull off the air hose and push open the butterfly in the TB. If it is black, it is dirty, clean with a small amount of petrol and a tooth brush and sorted in 5 minutes. Did this to my m8's Golf and it worked a treat on that also
Check with the servicing Vauxhall dealer that the latest ECU program has been down-loaded. After my 2000 1.8 Astra had been updated at 30,000 service (mid-'02) it ran much smoother and overall fuel consumption was up more than 2 mpg.