Help!! my polo is still missfireing

[low-and-slow-polo]

hi there i bought my polo with a missfire thinking it wouldnt be too hard to fix ,
I have changed one faulty fuel injector.
The distributor the rotor arm had a serious wobble so i had that changed.
All the spark plugs are fine and so are the HT leads.
I am about to change the coil can any one suggest anything else that can be makeing it missfire.
also when i turn my lights on the car nerly cuts out <- can any one suggest what this could be ?

Thanks Gareth