I've had issues with the same off favors in the past but have managed to do away with it for awhile now. I now keg most batches and I have never had the plastic off flavor in my keg. I got rid of plastic fermenters, vinyl tubing, plastic racking canes, etc. I use stainless steel or glass where I can. All my hoses are now hi temp silicon. I do still use a cooler as a mash tun though. I use star San religiously and clean everything very well prior to storage. I pay close attention to ferm temps and oxygenation now too. Been making some darn good beer!!!! Good luck.