Not some much unreliable as not the right size and style.

You should be using at least a 180°F thermostat for the D-jet to work correctly or it will run rich.

Is your fan clutch working correctly? Over time the fan clutch will slip too much and the fan blade isn't spun fast enough to move enough air to cool the water in the radiator.

If you radiator is truly original and has never been recored, I would suspect it might need to be serviced/rodded or replaced.

I haven't tried 1 yet, but they are making all aluminum radiators.

I am an old stick in the mud in that I like to use the old style original brass cored radiators in my customer's cars, but they now cost more than the all aluminum ones. What I am afraid of is when it is time for repairs and there will be a time for repairs, my radiator guy won't touch them as they don't weld aluminum.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502