Just had in identical car 2001 Astra G Z16XE with PO 300, 301 302, 303 codes logged, running like a dog. Codes read as above, had a genuine coilpack in stock, which I tried because this is a common fault and has cured some but made no difference on this one( Intermotor coil fitted ) so looked a bit deeper, on cranking compression felt down on at least one cylinder but did not have my comp. tester with me to test, and two plugs were oiling up in a very short time putting MIL on,on driving MIL flashing which must be when injector shut down to protect cat?This was clearly an engine mechanical fault as using borescope the bores were saturated in oil.

The cure,2 cans of Forte gas treatment in the tank (quarter full, just right!) Forte engine flush into engine run at 1500 rpm for an hour,changed oil and filter 5/30 Ford oil used,and a top end treatment added as well engine was totally different, however, on live data at 3000rpm b1s1, o2 sensor flat lined, You have already guessed, electrical noise from Intermotor coilpack was upsetting switching, fitted the genuine pack that I had in stock and threw the Intermotor unit over the hedge and car was transformed ! Forte call their products "liquid engineering" , in this particular case what a result, this car has done 108000 miles and has had numerous parts fitted at several garages at great expense and a few additives do the trick, hope this may help others with similar problems.