1997 GMC Suburban Coolant temperature spike

Frequently, the coolant temperature spikes to near overheating, but then quickly returns to normal temperature. A friend says that there is likely an air lock? Or some air trapped in the system. Is that likely? How do to correct it? This has been going on for several months.

Also, very recently, the air blown into the car suddenly was not heated. I believe the computer light came on, probably around the same time. My thinking is to replace the thermostat and extract the code at the auto parts store and go from there, but maybe you can give some diagnostic tips to someone without a lot of experience.