If it is the X18XE engine, EGR (exhaust gas recirculation) valve problems seem to be common on these. The valve should be closed at low revs and open in steps at higher revs.
When they get dirty they stick open and,at low revs, cause poor running.
If you can wield a spanner there is a good 'how to' on the following site (copy and paste)
http://www.astraownersclub.com/documents/technical/EGR.pdf
There is also a common fault with the earth from the MAF/IAT, but that usually puts on the EML. It needs a new earth from the yellow wire at the MAF to a good earth point, google for more instructions.
Of course if it's not an X18XE1 engine I have , once again, wasted my time typing all this out because people dont bother to post full details about the car (despite perfectly clear posting instructions on the 'ask a question' form). T'was ever thus.
vaux, March 2011
