Monkey puzzle tree problem: yellowing branches

Hi, I'm after some advice. My parents have a monkey puzzle tree, approx 30 years old. It now has yellowing branches and the problem is spreading up the tree, but affecting one side (the east side) more than the other. It is not the normal browning and die back of the lower branches as the tree grows. It is happening too quickly and is affecting branches higher than healthy ones. There is also some sap oozing from the trunk and on the branches in some places, but this is also the case with branches that appear healthy. Prior to this the tree has been extremely healthy specimen, growing well. It still has healthy new growth from this year and has (male) cones It stands in the centre of a lawn, but with a permeable membrane covered in wood chip to a distance of around 4ft from the trunk. It is in full sun and on a clay soil. Any advice would be most welcome as the tree is very precious to us.