SH submitted this idea :
Over typically 5 minutes, the air pressure will decay slowly to 80 psi
whereby the compressor motor restarts.

This happens when I am not using any air tools.

You have a leak somewhere, check the drain valve at the bottom of the
tank; any airlines you have attached. Tap water, with a drop of Fairly
liquid and a soft paint brush makes finding a leak easy.

My DIY garage compressor normally holds full pressure for days, with
nothing connected to it.