Supermarket contaminated fuel

Has anyone found after putting petrol into their car it turns into a kanagroo?...My taxi was running 'perrrrrfecttttt', very smooth, Mondeo, topping up with Tescos fuel, almost immediately I was dropping power on roundabouts, missed fired, and generally an unhappy engine. After discussing this with my garage who had recently put new plugs they said maybe dirty fuel and to get a fuel cleaner, I did, that helped but the problem persisted. In between supermarket fueling I do BP, Shell etc,. Then I bought Redex soft value enchanced additive, it help 100% but I have to keep this up. I now don't use supermarket fuel. My mondeo hates it. I also had a customer who picked up on my kangaroo taxi and said 'they wash out the tanks with water'.! Also other family members have expressed problems but theirs seemed to have resolved themselves. Maybe its just unlucky to get contaminated petrol once in a while like I did but found the same with Asdas. Any comments please?