I have looked around on the site and haven't really got a good answer to my problem.
A couple of months ago my Astra 1.6 SXi (2004) developed an occassional misfire, which got worse in time. I took the car to a local garage and there was no fault codes. After a while I got fed up with it and removed the spark plugs to see if there was anything wrong; all seemed fine and so I wiped the plugs and put them back in. The misfires went away for about a week and then came back.
This time I noticed that there was a vacuum at the dipstick, which after some research I figured that this required a new cam cover. So this was done and afterwards the car ran much smoother and didn't misfire. However after 1000 miles the misfire has now returned.
The idle is fine and doesn't blip like it did before the new cam cover was fitted (there seemed to be a bit of pressure in the engine and the throttle would blip every 10 seconds or so). The misfires only happen under load and do not happen all the time.
So, from my understanding this could be the coilpack, but wouldn't this appear as a fault code? How about the EGR? The misfires seem to go away for a while after the top of the engine has been taken apart (such as checking the plugs or changing the cam cover).