1998 blower working, heat works randomly

1998 Eddie Bauer. Turn heat on, blower work fine, but only cold air. Even after car is warmed up. Anywhere from 10 to 20 minutes later, it will start blowing hot air. Sometimes it can run for an hour with no hot air. Looking for some advice before I bring it in to a shop. I am new to town, and don't have a mechanic I trust yet. Thanks!