Howdy,I just got back from sand dunes and while driving with the A/C off and the vent on,thermostat on coldest,the air temp would be normal for about 60 seconds then blow hot for about 5 seconds.It did this the whole trip.I turned the t-stat up and down many times to no avail.I could use any help on this one.,,Thanks

 

I'm not familiar with the vent system on the '99 but it sounds to me like a temp mix door opening or closing. I'd start looking at vacuum lines under the dash to see if one is loose or cracked.

 

You could have one of two problems. Either the vacuum pump is starting to fail and allowing the system to cycle on its own when the vacuum drops too low, or the mixer door vacuum motor on the heater is failing. You have to get under the right side kick panel and get access to the vacuum motor and mixer door, but you can easily check the vacuum pump from the engine compartment.