Five months later, and problems with my car overheating are showing signs of returning

To answer the last question, only pink stuff went in the radiator. The last time the problem was a stuck closed t-stat that was changed. Original VAG coolant was used with distilled water to top it up.

Now the car is fine whilst driving, but stationary for 10 minutes and the temp starts to rise again.

If the t-stat was the culprit again, I can't image that happening again, the temp would go up continuously. I just got back from a 1300 mile round trip without any issues, so I'm completely lost as to the cause of this problem. Again, all three fans are working.

Any idea? I'm at the point where I would be replacing the car had there been more job security.