2002 Buick LeSabre air cond-heater problem

[jbhitar]

When ext. temp. is in mid 70s or lower, the initial start up, whether heating or cooling will bring the inside temp to the thermostat preset. Then, when the thermostat calls for more heat or cold, it will not allow air circulation (blower door won't open). You can hear the blower roaring behind the panel but no air is moving out of any ducts. If the ext. temp. is above 80 the cooling mode continues to operate normally.
When the problem is present, we turn off the system for 15 mins or so, then turn it back on and it will operate until it reaches its preset then shut down, but when it's time for more heat the blower runs but no circulation. This has been with us since purchased new and dealership hasn't been able to solve it.
Anyone ever encountered this situation?