Climate control problem on '98 Limited

I've been suffering through this problem for several months now and I'm tired of dealing with it, particularly in this 90+ degree heat. My dual climate control system has been working very sporadically. Most of the time the fan never comes on, but occasionally it will suddenly turn on and blow the appropriate temperature (hot air in cool weather, cold air in hot weather), though usually only through the defroster vents. Sometimes it will switch fan speeds by itself, sometimes not. Once in a while it seems to turn on as a result of hitting a bump in the road, other times it comes on as soon as I start the car, and even other times it may turn on after driving for 20 minutes and for no apparent reason at all. In any case, I seem to have very limited ability to affect the climate control with the control panel though I can turn the system off via the "off" button. Not being a mechanic I have suspected the culprit to be the control panel in the dash, but I dont want to start replacing parts unnecessarily. Can anyone give me an idea of what the problem is?
Thanks so much in advance.

As far as the blower goes, I would suspect a loose or corroded electrical connection at either the blower control module, or the blower itself. If all that checks out, it could be the blower control module. Blowing air out the defrost vent is usually due to a vacuum leak at the vacuum reservoir or one of the lines collapsing in the vacuum connector in the programmer. Check some recent post on AC Here (http://www.automotiveforums.com/vbulletin/showthread.php?t=419903&page=2), for more details.

[QUOTE=Leadhead] My dual climate control system has been working very sporadically. Most of the time the fan never comes on, but occasionally it will suddenly turn on and blow the appropriate temperature (hot air in cool weather, cold air in hot weather), though usually only through the defroster vents. Sometimes it will switch fan speeds by itself, sometimes not. Once in a while it seems to turn on as a result of hitting a bump in the road, other times it comes on as soon as I start the car, and even other times it may turn on after driving for 20 minutes QUOTE]

Z28 has listed the blower control module which is on the box above the blower under the hood, connections, blower motor itself, all as possible problems. That's the place to start. My blower control module went funny a few years ago on my 98. The blower would run at odd speeds for the setting inside, sometimes not run, sometimes change speeds after running okay. Those are the most llikely places to look for trouble.

But the part about changes when hitting bumps has me curious. I had replaced my programmer box (the part under the glovebox area that actually switches the vacuum and provides the logic). I've had minor light problems from the dash switch portion. I suspect connector next to the programmer is the problem.

You also might have a problem with the grounds that some people have corrosion problems on. I don't know where the one is for the dash panel and the programmer--There is one in the left panel by the parking brake. You might try banging on the cables you can see back there to vibrate them and freshen up a corroded connection. That's more likely if you're in a damp climate where snow or rain may have kept the carpet wet from getting in and out. (There are two other ground connections beside the driver seat and passenger seat under plastic panels flat on the floor between the seat and the door sill.)

Thanks for all the replies, guys!
I just love trying to track down an electrical problem. NOT. However, let me clarify that the "hitting a bump causes it to turn on" scenario is a rare occurence. It has happened a few times over the last few months, but normally the blower comes on at random, and it doesn't seem to cut in and out as though there is a bad connection. Rather, when it does decide to come on it will work for maybe 20 minutes then sometimes will quit again. Also, let's say I'm driving along and the blower comes on. If I shut off the car and restart it, there's about a 1% or less chance of the blower coming back on again, so that causes me to think it might not be a ground issue. Am I likely correct to assume that?
Back to the defrost vent, I have one other question. Before this started happening, for a few months I noticed that when I would turn on the heater or air conditioner the air would always come out the defrost vents for maybe 5 minutes before switching automatically to the selected vents. I can't remember if it always did this or not but I don't believe that it did. Is this normally how this system is supposed to operate?
Thanks again for all your help.

No, a five minute wait is not normal. Still sounds like a vacuum leak to me, but it could be an internal vac leak in the actuator. Need to make sure the actuator is getting vacuum, I'm guessing it isn't, but if it is then the actuator is bad.

Ok, I thought that was the case but wasn't sure.
Anyway, is there a way to troubleshoot the programmer, or is it a matter of swapping it out and seeing if it'll fix the problem? Not the problem of the 5 minute delay because I can live with that, but rather my main problem of heat and a/c not working at all.