That could well be playing a part. I would say the best technique is to flush once with the decent stuff, then flush once with water. I would suggest then fill with water and run the engine and do the airlock removal, this then should flush the contaminants around the system and dilute them, then drain this, and flush cleanly once more, then at the end top it up with the good stuff.
These combinations should suffice to make sure that this isnt playing a part. The only problem is that if the rad is blocked and the contaminant is on the other side then it could happen again.
I would probably suggest that in order to prevent a great investment in a new rad, call around a few scrappies and get a used one. Before you do this though, I would check all the wiring carefully, the stat housing, the coolant tank. Last thing I would want to say is buy a new rad to find that its something else.