1989 Nissan Truck 3 blown head gaskets

We have a very well maintained 89/90 nissan hardbody automatiic with approx 58,000 original miles. Never any problems until:

A few years back had a at approximatley 45k mi we experienced a blown head gasket, had repaired, including head milled, full deal while open.

5 to 7 thousand miles later, another blown head gasket. Discussed with repair shop they said the water journals in the head, and I believe the block were all blocked, appeared to be corroded. They cleaned out replaced replaced the head gasket, good as new.

Another 5k mil, same thing, same diagnosis, mechanic "reamed" out the journals this time, replace the head gasket and runs great.

Another few thousand miles and it blows again. Each time no symtoms, no warnings, no smoke, no overheating no water in the oil then just blows.

Before we get it fixed again I am wondering is their something inherent in that years engine block or head that would cause the journals to clog so quickly or at all for that matter? Could this be a radiator or even an engine coolant issue or some sort of chemical reaction?

Oil has been changed every 3k mi since new, all other fluids changed religiously all tune ups etc. Done on schedule, low miliage, before we go through this again need some help. I used to turn wrenches but that was years ago, just want the right diagnosis so I can get a qualified mechanic to fix.

Thanks,
Tony F.