98 RS
I had a used radiator installed in my car, about a year ago, and I been having the issue ever since, the coolant keeps getting dirty (oxidized), it comes out brown, I took it to a place to have my system flushed, and that worked for about 2 weeks,I bought radiator flush bottle from an auto parts store, and that worked almost 4 days, and the oxidation came back, I flush the system at least once every 15 days, using only water, and it comes out lightly dirty, just to keep the car moving.

What can I do to get rid of oxidation once and for all:
I've found on the 'web' solutions that I'm thinking to attempt:
-Flushing the system with white vinegar.
-Taking the water pump out and cleaning it, changing the thermostat, and taking some chemical (again; radiator flush) and flushing again.

For people who have had this issue, what has worked?
Should I just go to a different shop to have the system flushed again and start from there.

"Imagination is more important than knowledge." Einstein.