You need to watch the unit while the shower is running to see if it is producing any error codes. It might be overheating, and shutting itself down until it cools off. There are other reasons, the manual should help here on that. If you don't have soft water, you may need to do a descaling of the heat exchanger. To produce the hot water on demand, it needs an intense burner which, if you don't have soft water, will produce some scale as it is being used. Eventually, this can partially clog the passageways, and lower flow with the same heat input...the water gets too hot. Depending on the shower valve you have, you may not notice until it shuts off.
If the install included the needed valves, it is fairly simple to run a mild acid through the heat exchanger to disolve the mineral scale. You need some hoses, fittings, a pump, and a bucket along with the acid (some use vinegar). The manual should show you how to do this, or a plumber versed in tankless systems could do it for you. nearly 10-years is more than enough time to have scaled up to produce that problem...some manufacturers recommend doing it annually.
Personally, I don't find tankless that great. Most people don't want to do the required maintenance, and they have their oddities, plus generally cost more to install. Depending on use, you may never recover the increased installation costs with operating expense reductions. Plus, finding someone who understands and can service the things can sometimes be tough.