Well, I guess you've stumped us. You should quit brewing.
Seriously, I can't see what we are missing. I would recommend stop using the acetic acid. Use Star San or Iodophor. Acetic acid even in minute amounts can cause the sourness you are describing.
Plate chillers can be a cause of infection, though it does sound like you properly tackled sanitizing yours. But you may try an immersion chiller.
Aside from that, it is really difficult to infect a beer as badly as you are saying yours are getting. I mean, you either have a super bug, or you have a really dirty piece of equipment you are over looking, or you are misdiagnosing a fermentation problem for an infection.
You are fermenting a little warmer than I would recommend. Try cooling your wort down to the low 60s (F or about 16C) before pitching your yeast and keep the fermentation temp, which will be several degrees above ambient, below 68. If you have a fermentation chamber then the temp of that chamber should be set to 60 to keep your temp in the proper zone.