AC/Heater...blows cold then hot

:banghead: I have a '99 blazer with V6 moter. Car went dead today...replaced battery. Now A/C blows cold then hot...can hear vent cycling back and forth....recently (about a month ago) had intake gaskets replaced but don't think that has anything to do with the problem...A/C compressor seems to cycle normally...any clues....help
:banghead:

Automatic climate control or manual? When was the A/C serviced last? Does the heater maintain a constant temperature? Is the engine temperature gauge reading approximately 200 degrees and stable? When the engine is running, can you hear a hissing sound in the dash or just the "vent cycling"?

It is climate control...I can hear the door open and close that allows the air to flow thru the heater core. It just opens and closes about every 5 to 10 seconds. No hissing...I was told it probably isn't vaccum operated but probally has servos....don't know about that though...
temp gauge stays fairly steady...A/C compressor seems to cycle fine (stays mostly on)....just that vent opening and closing...hot then cold about every 5 to 10 seconds
Thanks in advance

Automatic climate control or manual? When was the A/C serviced last? Does the heater maintain a constant temperature? Is the engine temperature gauge reading approximately 200 degrees and stable? When the engine is running, can you hear a hissing sound in the dash or just the "vent cycling"?

The automatic HVAC system is rather complex...
There are 4 temperature sensors, 1 sun load sensor, 4 vacuum operated actuators, 1 electrically operated, (feedback) actuator, 1 powered aspirator, control switches and a control module, (and probably some things that I forgot to mention). The control module collects data from the sensors and the feedback actuator, along with data from other vehicle systems that are interfaced with the automatic HVAC system and, if operating correctly, does what it was designed to do ;) If not, a body capable scan tool is required for diagnosis and repair.

There are a couple of things you can try:

To recalibrate the system: Turn the key to the OFF position, remove the RDO BATT fuse for 10 seconds and then reinstall the fuse. Turn the key to the RUN position and wait 40 seconds for the module to initialize and recalibrate. One of the things it recalibrates is the feedback actuator.

The feedback actuator operates the door that you hear moving and it is located on top of the heater case, directly behind the glove box. You might check the connection to it and make sure it's tight, and also check all fuses. If that doesn't take care of it, it's time to call your neighborhood GM dealer.