The main culprit for flapjacks that fall apart or disintegrate is cutting them before they have fully cooled. If you are impatient and want to dig into your prized treats as quickly as possible then leave them on the counter until slightly warm and then transfer them to the fridge to fully cool.
It is also important to press the mixture down firmly before placing them in the oven. This will help to press out any air and help everything stick together.