Why does my hot water go cold after 5 minutes?

When i try to run a bath the hot water goes hot rather quickly then goes ice cold and after another 5 minutes or so goes hot again for 2 minutes and so on. It can take over an hour to get enough warm water for a bath. Yet my central heating goes hot and keeps as hot as i want it. Please help. What can i do to fix this problem. My combi boiler is a Vokera Compact