One thing I note in the OP is that he was using an Acid sanitiser as part of the process, isn't any more and is now having problems.
Beer Stone (Calcium Oxalate) will build up and it provides a great refuge for bugs, it takes an acid cleaner to remove beer stone, all the hot caustic, percarbonate, bleach... in the world wont shift it. Acid sanitiser and Iodophor will (Iodine and phosphoric acid).
There is a decent possibility that that's what's missing from his cleaning regime.
That said persistent infections can be a bugger, sometimes the best you can do is to no-chill a couple of beers, if the cubes don't swell at least you know the problem is down stream from there. sometimes it takes a complete replacement of all your plastic (though bathtub soaking in bleach as above should be pretty effective), sometimes you can track down what is called the seat of the infection, be I a transfer hose, even seen plastic fermenter lids that weren't well made and were porous. I remember one customer who couldn't brew in March, there was a big tree next door that when in flower was shedding masses of wild yeast, the air itself was a seat of infection, so clean room (as is in filtered air) or skip brewing in March...
Good luck, I know just how frustrating it can be.
Mark