Another thought to consider: is there any other variation to your brewing routine? ANYTHING!
At one point I had (seemingly random) plasticky band-aid flavours, but only in some batches of beer. Drove me nuts for ages. Eventually tracked it down to a non-food grade hose that was sometimes used to fill the mash tun and HLT with water for brewing.
Hah, wow. That would have driven me nuts as well.
Only recent changes:
Put a 2 piece ball tap into my brew pot to change it into a kettle with a no-weld fitting. I take apart the no weld components without disassembling the two piece tap itself between brews; percarbonate soak all the pieces - usually wrap new teflon plumbers tape around the connecting screws and spray stellar san into the cold side prior to transferring the cooled wort to fermenter. This change does precede the bad batches but there's no consistency - plenty of good batches along with these two bad ones. I'll be more careful when setting the tap up - wear gloves rather than handle any of the components with bare hands during assembly. Couldn't be the teflon tape could it - maybe some exposed on the hot side? But seems unlikely given the strength of the off flavour in the first bad batch. It tastes like a funky belgian but turned up to 11. No excessive carbonation at this stage but I did put them into the fridge relatively early on.
The hose I use to transfer is clear food grade tubing from the brew store.
Other change was hot side - stainless steel basked to augment the mash bag and make sparging/mash out about 50 times easier but that is pre boil and also used for several good batches.