Continuous forced resets

Hi guys,

I don't know if I'm past saving, but I'll try my luck:

Since a year and a half I've had a Sony Xperia Z5 compact (Android 7.1.1). For 9 months it has been getting hotter and slower. This is still fine with me but recently it has started to continuously reset itself at a certain degree of utilization (which is reached quite quickly), usually 10-40 minutes long. So I use it maybe 10-30 minutes (sometimes one, so I'm not sure about the link to heat), then it keeps resetting itself over and over, sometimes over an hour.

Do you have any idea why this may be? The hardware shouldn't be so old to be this overwhelmed, right?

Many thanks,
Chris