hot water goes cold

I've got an annoying problem with my continuous flow (instant) gas hot water system. Every morning when having a shower, I get nice hot water for about 2 minutes, and then it goes dead cold. After about 30 seconds it then comes back to hot, and stays hot for the rest of the length of the shower. I don't know what's going on... any suggestions? thanks, gav
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What make my Bosch came with a good trouble shooting manual. Take off cover, run water and stay at the heater to see is it shuts down or anything noticable happens. It could be several things. Bosch also 800 phone troubleshoots well.
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flow switch sticking???
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I stood in front of the unit when someone went into the shower, and the flames just died off, which left just the pilot flame on. After a minute, the flames came back again.... ?

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I also forgot to mention, our gas heater often shuts itself off. Could there be something wrong with the gas supply?
thanks, gav
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