First step: Attack it with science: is there anything common to the smelly brews that isn't involved in the others: same fermenter? same shop the kit was bought to (i'm assuming you're kit'n'kilo'ing?). If there's something you're doing differently in the smelly brews than the good ones, no matter how subtle, that's where you should look.
My other thought, as it's the only time i've ever tasted a rubbery plastic smell, is: are you getting water through a garden hose, or some other kind of not-totally-food-quality tap or attachment? This is pretty simple to test actually - go grab a glass of water from whereever you fill your fermenter.... does it taste weird?
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Have tried to rule out any changes, but cant seem to think of any? And as i said it is happeningEvery second brew or so???
Cheers