Looks like we've got quite a discussion going here
I don't have anything new to report, just wanted to say thanks for all the thoughts and to respond to a couple of them.
I know that not being able to examine the situation first-hand leaves a lot of room for questioning, so maybe, if you can stand it, a bit of rambling from me might help.
I understand the thought about it possibly being a partial blockage in the heater core because I had the same thought earlier on. I also came to about the same conclusions. Unless the venting is separated, the hot air would mix and all air would become "warm", instead hot/cold right/left. Of course, being able to actually feel and observe the differences in temperatures of the air coming out of each side of the cabin gave me an advantage.
Once the radiator is warmed up, the air from the passenger side is VERY hot. The air from the driver side is VERY cold. Not warm... cold. There is no apparent mixing at all of the hot and cold air. If it was warm enough outside to tell if the a/c was on, I would know for sure, but if it'* not on, the air coming from the driver side vents is certainly cold enough to think that the a/c is on. To me anyhow. But one can never rule out any possibility, blocked core or whatever, until a cure for the problem is found. It'* just that the odds seem against it being the core.
I plan to measure the difference in temperature coming out of the vents with a thermometer this evening. Will post it tomorrow.
I agree that there hasn't been any testing to determine the strength of airflow and it'* certainly a reasonable point. I can't think of any place to readily get my hands on the equipment to measure it, so again, it falls back to being able to observe it first-hand. Being a Bonneville Club, I'm sure everyone here is as aware as I am of the vent locations on the driver side. IMHO, a very poor design. In the summer, blows all the cold air directly on the driver'* hands. I've learned this week that even with those two vents closed, it still leaks some air on the drivers hands. Mine were "frostbitten" this morning after driving only a few miles.
Certainly Randman is right about airflow being less, the further away from the fan and this slight difference can often be felt at each vent as he describes. Disregarding these slight differences, the airflow feels equal at each of the 4 dash vents. It also feels every bit as strong as it did before this problem came up. It'* only my opinion, but I feel confident in saying that the airflow does not seem to be hindered in any way. That seems to be true at any fan speed.
Closing the dash vents on one side to see if it forces the air on the other side to mix is a good idea. I didn't think of that this morning when I closed the driver side vents, but I will test it this evening. I'm not sure that I'll be able to tell if the passenger side air cools down, but I'll certainly try. I'll also reverse it and see if the cold air on the driver side warms up a bit. That might be easier to feel. I'll let you know.
Thank you for all the thoughts and posts you've sent. The info you've provided on tracking down vacuum leaks and on the vacuum control modules has been very enlightening. If Tim (98bonnie) doesn't find the problem before I have the time (and a warmer place) to attack it, I'll certainly keep you posted. This is a frustrating thing that needs an answer, for everyone.
You're a great bunch and I'm glad you're here.