On one of those trips, if you shoot the rad imput/output temps when hot, and we know the new rad is doing its job of shedding 50*ish, then we can eliminate that and try other stuff.
If the rad is 180* ish at the top, and 170+*ish out the bottom, that would be a problem.
The blow back is kinda like the pumps doing its thing creating pressure while running, then when shut off, the pump pressure shuts off, and surges back.
We need some help here.