Honda Civic A/C blowing hot

I have a 2003 Honda Civic LX. A few weeks ago it started blowing hot humid air. This will last for about 15 minutes, then I’ll feel an air pressure change, slightly less pressure. About 30 seconds later the air starts blowing cold. If the car is shut off and started relatively soon the air works normally. If the car has been off for an hour or more, the hot air starts again. Any ideas.