Could excess water in the boiler cause overflowing radiators?
Two common causes of overfilled boilers are a defective auto fill valve (if equipped) and the tankless coil. You can test the auto fill valve by shutting off the supply line to the boiler. If the overfill condition ceases the auto fill is the culprit. If you have a tankless water heater (a coil inside the boiler that makes your hot water) it may be leaking. The leak allows uncontrolled domestic  water to enter the boiler causing the overfill condition.