I'd begin with that premise. Flush the repaired radiator and system real good, and refill with 50/50 mix. Run it a day and check it, If it looks OK, keep driving it as usual but check the coolant before your start it each day.

You'll know after a few days if the problem still exists.

It really sounds like some kind of oil in the coolant, and that can happen at a couple of spots inside the engine, but that's an extremely rare occurrance.

Keep an eye on the oil consistency, too. If it starts looking gray or creamy, you've got a problem.

Hmmm...Does this car have an automatic transmission? With the cooling coils inside the bottom radiator tank? It's possible that coil may have been damaged when the radiator was repaired, and you're getting automatic transmission fluid into the radiator.