LM393 output flickering?
Are you connecting this directly to the car battery (12.6-13.8) V? You are going to see huge transients on that. The transients will cause unstable operation and/or blow this up.

Try turning on your AC. Probably will see interrupts happen. You need to design some kind of circuit to protect the sensitive electronics from all the large automotive electronics (AC, alternator, crank motor etc.)

Read this article here that explains how brutal the automotive environment is to electronics. TI also has a great article here.

Image from Lite-fuse Article mentioned above.