Could well have been a badly maintained cooling system. Unlikely to have ever been flushed. Maybe different types of antifreeze had been used in the past. There are now several types on the market some which shouldn't be mixed with others may have caused a chemical reaction.
Now you have flushed the system, did you keep the heater on hot to flush the matrix? Check the thermostat in some hot water and check it opens and check the cooling fan is operating. Hopefully it's now sorted.
An oil change, new filters and spark plugs if not already done will be beneficial.
