Considering the age of this model, it sounds like CPU is overheating. Causes are dust and the CPU thermal paste is bad and needs replaced. Again, considering age the thermal paste is the most likely as it will deteriorate over time.
Here is the Dell manual (although it has very little detail). Remove the CPU cooler assembly, clean off the thermal paste from the top of the CPU and the bottom of the heat sink. Apply new thermal paste (Arctic Silver is recommended) per the thermal paste vendor's instructions and reassemble.
Dell forum member since 2002
Dell Inspiron 15 - 5577 Laptop
Home Built Desktop PC with ASUS ROG Z170, i7 6700K CPU
Windows 10 64 bit Pro and Windows 10 Insider Program Beta Versions. SSD drives. Cakewalk by BandLab and Studio One 4.1 Recordng Studio Software.
Dell S2719dgf Monitor
Member of Nashville based R.O.P.E.
Models from this Vintage can have clogged heat sinks and thermal sensors as well as Bad Capacitors. The price in labor without any parts to fix that problem is more than the unit is worth. Getting a new working unit with windows 10 is easy in the USA. I refer to these as free computers because the cost of windows 10 is the same price as the unit with windows 10.