It could be anything so correct diagnosis is needed, has it been losing water? and if so how much and when. If it is losing water you need to identify the leak.

What condition is the cooling system in? many of these vehicles become clogged and blocked by years of abuse such as a lack of anti freeze and people throwing dirty water in, or just failing to clean their cooling system periodically. The solution is simple, clean your cooling system.

Remove your bottom hose and drain the cooling system, ensure your heater is set in the hot position so your heater matrix also drains, flush out the radiator with a hosepipe and then flush out the engine block with the hosepipe, this will remove a lot of loose crud and give you an indication of your cooling system condition, if rusty water comes out, or a lot of debris comes out then clean it properly.

Fill the cooling system with water and run up to temperature and drain it again, any loose crud circulating in the system will now be drained, fill with clean water and a good cooling system cleaner, follow the instructions for the cleaner, drain out and flush with clean water again, noting what comes out. Replace the thermostat as a matter of course.

Refill the system with a 50:50 mix of water and anti-freeze, and I always recommend adding a bottle Bars as this lubricates the cooling system.

If your problems persist you need to be looking at the water pump, blocked radiator, or a seriously contaminated cooling system issue.

These engines are known to have issues with a web internally in the cylinder head corroding which leads to a cracked head, this is difficult to diagnose and even a pressure test often fails to find it, so this is a possibility, but eliminate all other causes first.