I can commiserate as I’m on my third rad in my car and its quite frustrating. A rad may look brand new from the outside and have no mounting issues but don’t rule out the electrical killers from within. For instance if you have an iron block with an aluminium rad and old coolant this can cause galvanic issues, or stray currents in the coolant system can cause electrolytic corrosion. Both can attack the core or braze/solder from within. Even parked in the garage it can be quietly rotting away. If you have a multimeter attach the negative probe to ground and dip the positive probe in your coolant. Any voltage say over 0.1 volts could be cause for concern but if you have more than 0.3 volts it can indicate a significant issue (check with engine running and not running).
