99 blazer with the auto temp controls.

Recently when the temp has been set to max (84*), it has been blowing cold air and then atarts blowing hot air for a while only to start blowing cold air again. This is after engine is warm.

Any ideas on whats causing the problem?

Sounds like you may be either low on antifreeze, If your radiator is full then your thermostat my be sticking open...