Found on our forum:

Astra Engine Missfire
Can anyone throw some light as to the cause of a loss of power during acceleration , 
the fault only happens when the engine is warm and feels like it has gone of one or more 
cylinders . Once the throttle is backed off , full power returns .No fault lights appear , the fuel 
filter has been replaced along with the plugs, and a dose of injector cleaner has been added. 
any ideas?

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Try Checking the coil pack. 

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Would a main agent be able to test the coil pack ?

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I am assuming this is a 16 valve engine 
Yes they can,  
    but if you have some mechanical knowledge and four spare plug leads you could do it 
yourself, switch off the engine, unbolt the coil pack that is the long box sitting on top of your plugs, 
remove the wiring plug at the right hand end (press in the spring clip and pull), gently prise off the 
coil pack and turn it to reveal the plug connections remove the rubber sleeves and put the plug leads 
one to each coil tip and connect the other ends to the plugs, make sure they are secure, plug the 
wiring back into the end of the coil pack. 
 
        WARNING. 
If you feel at all unsure of all this, please just take the car to a garage I will accept no responsibility 
what so ever  
 
Now get a spare spark plug, take the first plug lead off at the plug end and fit it to your spare plug, rest 
this plug onto a good secure earthed position  and start the engine, note the spark quality switch off engine, 
reconnect plug lead to original plug and repeat this for the other three if it is a faulty coil pack one spark 
should be a lot weaker than the others if it is not then take it to a garage and let your wallet do the repairs  
Again please take care you, are dealing with 15,000 to 20,000 volts and it could cause injury.

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Problem solved , did as you suggested , and found the spark from No 2 coil much weaker , however this was 
only evident when you raised the plug from the engine block . New coil pack cured the problem.

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