From what I understand, you don't really know the history of the car. If it were me, I'd flush the system once or twice more. I'd use distilled water, GALLONS of it. Then, the third time, I'd add distilled water and coolant and see if it comes back. Perhaps there is contamination from a past 'incident' and all ya gotta do is flush/clean it out.

I sure hope thats what it is. Is the oil looking OK? If it looks ok, seems to be it's something with the cooling system, but if the oil is contaminated, then I'd think about a head gasket, etc. a

I dont know how else to deal with it.