I have had the same problem. I have searched and searched forums and homebrew shops for answers, most of the time I get the answer as temperature as the cause. This winter I have been brewing and brewing and I have not had any plastic issues yet, but I know exactly what you are talking about. I will give you my opinion based on what I have been through.
The Brews I did last year I was not able to recover. The temp got well into the 80's much higher than my yeast could handle. I still have some in the bottle today and its horrible. My first brew this winter was a American Brown. When I transferred it to my secondary it had that plastic smell you are talking about. I was very upset but went ahead and went to secondary then to the bottle. The plastic taste has faded almost gone. I have made my own determination that hops are having something to do with this. I dont know if its using old hops, or hop oils or a certain kind of hop. I would be curious to know what kind of hops you used and how old they were. I would say if your withen range of your yeast temp your ok, but if its still has that plastic smell when you bottle or Keg I would say it will still be there when you are ready to drink it.