What brand was the thermostat that was in it and what are you using now?

Most of the aftermarket thermostats aren't exactly correct in the bypass department or a just junk.

The original radiator is still the best choice if the fins are still good, they are seriously oversized to start with.

Have a 2nd look at the bottom hose, sometimes they can suck inwards or a piece of the inside can form a flap.

70°C is far too cold at least use the correct one.