Suburban water heater - DSI fault light goes off and on

When turning on our water heater (we always use propane) the DSI fault light will come on, you can hear the flame burning and the fault goes off as it is supposed to. However, in a minute or two, the DSI fault comes back on and stays on.

We have discovered that if we run the hot water in the sink for a few minutes and then turn on the heater, the DSI will come on as it is supposed to, then it goes off and stays off as it is supposed to. But it's annoying to be wasting our water down the drain each time we want to have hot water.

Any suggestions?