I'm having the same problem. Before I assembled my loop I went overkill on the cleaning just to make sure no flux was left in the radiator. About one hour of flushing with hot water straight out of the tap, two soaks with 99% Isopropyl, 10+ shake sessions with hot distilled water, 2 soaks with distilled. To no avail I'm afraid. One month after assembly the tubing started to cloud and after six months it was completely white.

I disassembled the loop to try and clean it up. All parts of the loop had a white, powderish film which didn't dissolve with Isopropyl or hot water so I decided to see what vinegar could do. I put a piece of the tubing in a warm vinegar solution and indeed it reacted with the white stuff and cleaned the tubing pretty well. I cleaned the radiator using the same procedure as the first time but also added a vinegar flush since it did such a good job on the tubing. The result? Clouding after one week, completely white after a month

Six months later on I have once again disassembled to make place for some new parts and the white stuff now has company with rust coloured stuff I've used nothing but distilled + one drop of PT Nuke btw and the liquid itself looks fairly clean. The inside of the radiator however, does not. The ends of the tubes have a white film and the end tank is covered by something I can only describe as rust. When I read about people just cleaning their PA's with a few rinses of hot distilled without any problems I can't believe we have the same radiator.