phone restart automatically and improper sound occuring

I bought Nokia 8 in March 2018. After using it for one year, it randomly restarted 4-5 times in row. It will restart maybe once, or 10 times in a day. It is mostly restarting when I open camera app. Although this doesn't happen when I am charging the phone. It only happens when I am not charging the phone. There's also a sound issue where the sound freezes or is not audible when the volume is full. Again, it doesn't happen when I am charging the phone. I tried resetting the phone several times. Last week, I sent it to the shop. He first said that it's a battery issue and today he told me it's a motherboard issue and I need to replace the motherboard. My phone just lost warranty in march 2019. It will cost me 16k Rupees to fix my phone. I don't know if the shopkeeper is lying or not. Any help will be appreciated. Let me know if this issue is common in Nokia 8 and what should I do next. This is really unexpected from nokia. Please do needful actuac asap.
Waiting for your valuable response.
Thank you.