Try this, it will be your cheapest way of tackling this problem. Go to the part store and buy a set of NGK BKR6E plugs, they are single prong and are the best for these cars. They are $2.75 a piece (x4), so $11 plus tax. Yank those old plugs out and replace them, but when you reassemble, swap the coil that youre having the plug issue, with another one and see if the problem follows that coil. By doing this you have A) the correct plugs B) can determine if the coil is causing the issue and needs replaced and C) if it does not follow, you can start to look and see if the original cylinder has a head gasket leak and a leak down test would follow.

Edit* also the white smoke usually indicates coolant being burned off. Sort of need some more detail information to help, but hope the above gives you a place to start