Hot and cold water from tankless heater

Question

I installed a Richmond brand tankless water heater and the water runs hot for approx 15mins. and then goes cold. It takes 3-4 mins. for the water to warm up again. The heater is natural gas and we never had a problem with our old tank heater. Could there be something I did wrong on the installation? I followed the directions to the letter and have tried to pinpoint the problem with troubleshooting guide but with no luck.
Thanks for any help
shivering in the shower