You may have a loose vacuum hose either under your dash, or in the engine compartment.
Bad "blend door" actuator or stuck blend door on the driver's side.
Temp blend door actuator is bad on the passenger side assuming you want both sides to blow cold air.
same thing happened in my car regal as well and it was only once and went away when i started it again
because someone was fiddling with the passenger climate control and now that baffle has stuck in the cold position . I had a 1997 buick lesabre which had the climate control functioning until a friend started adjusting it a few times then it became stuck in the cold position but only for the passenger side It is likely a mechanical bind in the baffle .
I would change the thermostat first. If that dont work change water pump
Why does my Lincoln LS V8 Blow cold air on one side and hot air on the other side
Your freon level is too low. Have it topped up and dye added to check it later for leaks. I had a very similar problem with a 2000 Olds Intrigue.
One of the reasons a 2000 Jeep Cherokee heater would randomly blow cold air and then hot air and then cold might be that there is a problem with the hot/cold duct. The air or a vent might be inadvertently blowing periodically.
The most common thing is the heater / climate control valve located in the dash area for controlling the heat output. Should be able to tell if using the dual mode changes the heat output.
When my a/c system is low on refrigerant, it blows hot on drivers side, cold on passenger side of the dashboard. This is a 1997 Buick Ultra. Recharging the system solves the problem.
you need a new thermostat and possibly some anitfreeze
The heater might blow cold air in a 2000 Silverado because the thermostat is sticking and not changing from cold to heat. The thermostat is usually located in a hose that leads from the radiator to the motor.