The more I read this - particularly the original: car boiled and continuous bubbles from the bleed - the more I've got to go with HG or worse. Nothing else makes sense. I presume the radiator, water pump and thermostat all tested good?
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i