A/C Turns To Heater and Back Every 5 Seconds

I have a 1999 Chev S-10 Blazer LT 4x4 with a 4.3L motor, automatic transmission and 137K miles. The car has the climate control feature for heating and cooling. All of a sudden when you turn the A/C on it will run for roughly five seconds with cool air then turn to heat for 5 seconds and continue this pattern of changing between hot and cold air every five seconds. We checked the computer and proper signals are being sent to the A/C. We checked the climate control panel and it seems to be working properly with no shorts. Still the blend door keeps switching hot to cold. Any ideals as to what to examine next or what the problem might be? Thank you