Fill it right to the very top with the cap off and see if it keeps doing it. If it keeps having bubbles head gasket and or the head is not in good shape. Might have to start it a few times to get all the air that would be in there from doing the
thermostat. If that's all right might just be the rad. might need cleaning or replaced.

1997 Montero LS with a motor swap a 1990 V6 12valve ported heads full 2.5in Ex I run stock tire size for Daily use, off road have 33x12.5 working on going trubo.