Reoccurring gusher bug?

In the three years I've been brewing in our new house I've had several (5-6 of 16) beers infected with gusher bugs. I've switched from plastic to glass fermenters, started using starsan instead of that BS "no-rinse" stuff most rookies get conned into using as it comes with the kit, and even gone back to using carbonation drops to reduce the chance of infecting it while batch priming. Even so, my latest brew (Stone Levetation amber ale remix) is already tasting "gusher-y" (despite being dry-hopped twice with 6g/L Amarillo/Cascade mix, and the first bottle tasting bloody good, it has almost no maltyness and no hop flavour/aroma, just that thin, bitter, fizzy taste) and has way more fizz than it should. It's only had 15 days in the bottle, and already I have to be super-careful when opening and pouring to avoid it foaming everywhere, I wouldn't be surprised if in another week or three they'll be erupting as soon as the cap comes off.

I think I'm pretty careful with sanitation during the brewing/fermenting stage going by what I've seen others do when I've brewed with them, and in this recent case I even ran the bottles through a commercial steriliser at a friends cafe and sanitised with starsan before bottling. I soak the carboys in napisan between brews, then rinse and sanitise with starsan. Having just bought a grainfather and kegging gear to step up production, I really want to get to the root of this problem as it is starting to piss me off having lost so many good brews!

Firstly, what are the chances that it could be caused by some sort of airborne yeast/bacteria? I never had a problem in our old house, but our new place is set into the bush and I do all the brewing/bottling downstairs in the garage and spare room, which can be quite damp and musty at times, if not kept well ventilated and/or heatpump running. I'm pretty liberal with my applications of starsan, most things are soaked in it as well as being misted with a solution from a spray bottle if they've come into contact with anything or been out of the soaking bucket too long. Which leads me to the second question;

Once something like this has come into contact with plastic brew gear (siphons, spoons, wine thief, racking cane etc) how hard is it to completely remove it? Any suggestions for what to use for a real thorough soak/clean?

Losing this latest brew is a real kick in the guts, especially as it comes just after spending a fair bit of cash on gear for a hobby that I otherwise thoroughly enjoy!