2003 Holden Barina Fault codes PO300 and PO301

Computer problem
2003 Holden Barina 4 cyl Front Wheel Drive Automatic 80000k miles

Engine intermitantly miss fires. Check light comes on. Scan tool shows two codes. PO300 miss fire detected cylinder one. PO301 random/ multiple miss fires detected. Plugs and coil pack removed and inspected, no obvious defects found. Plugs and coil pack replaced, fault still present. Vehicle has come to me after being at two other workshops including Holden dealership, still not fixed. I have two Barina's now in my workshop a 2001 and a 2003. Both doing the same thing, the same symptoms, same codes. I'm guessing this is not a one off fault because I have two of them doing the same thing so I'm hoping someone has struck this fault before and can help me. Regards Russell