I have the problem of the oil pan filling with gasoline causing the oil level to rise to the point of backing up into the cylinders. This causes the plugs to foul and give the same symptoms as you describe, with the exception of it fouling both plugs instead of one. Maybe you have the same problem with less compression in the rear cylinder causing it to foul in the rear first.
Check your oil level, make sure it is not too full and monitor it carefuly. If it is too full then that maybe your problem if not it may your rings (compression check?).
Please keep me posted on your progress, I hope that this info will help fix your problem and maybe you will come across something that will help mine. I have had my carb rebuilt but it did not fix the problem. I am not sure of what to do next.