A decent rad and an electric fan will help prevent problems in the future.
We do a rad for modified Anglias which is a high density triple core with a fanswitch mounted in the bottom tank, this brings the fan in and out just like a modern car.
I guess two reasons for why it has 'suddenly' start doing it would be there have been some of the hottest days on record this year and also as time goes by your rad and block 'fur' up with rust particles and crap from the water, just look in a kettle. This restricts the flow of water and the cooling system isnt working at peak efficiency.
I know most antifreezes claim to have rust inhibitors in there but how effective they are is anyones guess.
Also if the rad and block are original to the car there is no way of telling how many years it was run without anti freeze, back in the day people only used to run it in the winter and ran ordinary water in the summer months.
Try taking the rad out and back flushing it and then also while its out try and give the block a flushing too, this could help, oh and do the heater matrix if you can.
Hope that helps.