The boost gauge is only an INDICATION of something. The BCA and BCS are not the cause. BOTH of my cars do that depending on engine load, temp, or humidity. Particularly in the cold mornings when air density is higher.

To the best of my knowledge, this is NOT a problem as long as your IDLE RPM'* are steady, and are what the PCM is requesting.

Having owned 2 supercharged Series 1 bonnies since '99, I know that something'* wrong.
Again, the only time when the problem is noticeable is when the engine is idling in Drive or in Park; boost behaves correctly otherwise, and there'* no stumbling or hesitation at any speed or under load. When the problem occurs, the engine surges up and down a little; although the tach shows only a small variation in RPMs above and below 750 or so, the surging is noticeable, and that'* how I first discovered the problem.

I strongly suggest you pull your TB for a thorough cleaning, particularly the IAC and the IAC passage. That'* the number one cause of a surging/hunting idle.

I cleaned the throttle body about 2 months ago; the problem still occurred. I then removed the supercharger and replaced the base gasket. I did this, and several other things, in an attempt to also fix a rough idle condition that seemed to be getting progressively worse over time.
After I replaced the supercharger base gasket, the rough idle problem seemed to be fixed for the most part, but the idle is still a little rough sometimes. The rough idle comes and goes; some days it idles smoothly; other days it idles a little rough. RPMs are always steady around 750 RPMs at idle, even when the idle is a little rough. I can live with the intermittent rough idle, but the occasional surging/boost problem is what I'm trying to fix right now.

Why aren't the boost bypass valve and boost control solenoid possible causes?
Thanks.

"All vacuum hoses look to be intact and connected properly."

You have an indication of vacuum loss on the gauge and a rough idle.

Since, in my experience, about 2/3rds of rough idle problems are vacuum leaks, I'd remove the hoses one by one and check them.

I'll bet you find at least one with an intermittent leak.

Do you get smooth, even boost at WOT or on the way there? Does your BCA fully bypass when at idle (you indicated it did earlier). They're working fine. Unless that actuator is fluttering at idle visibly, the boost gauge is just an indication of your problem. It'* reading manifold vacuum at idle, just as a non-supercharged car would read if you installed that gauge in it.

Thanks for commenting on this, Bill. I've been trying to figure this out for months now.
Yes, boost seems to be smooth with increasing throttle and at WOT. I couldn't see whether or not the BCA was fully open at idle this morning because it was still dark outside. It didn't look like it was fluttering, but I couldn't see for sure because of the poor lighting. If I had my scan tool with me at the time, maybe it could have given me a clue about the cause. Unfortunately, this problem has never occurred during the daytime or at home at any time.

So if manifold vacuum is varying this much at idle when the problem occurs, and it'* not related to the boost control system, what are the possible causes? I perviously thought there was a vacuum leak, but I never found one, and if there were a vacuum leak, wouldn't it be a constant leak that would cause a consistent symptom?

Vacuum leak seems the most likely to me. Vacuum leaks can be variable based on atmospheric conditions, temperature cycling (SC heats up faster than the motor), that kind of thing.

Spraying starter fluid or intake cleaner around and on each fitting can help identify a leak. Your idle will change slightly if it sucks some in.

On multiple occasions I've sprayed intake cleaner on things to try to find any vacuum leaks. Because the idle problem is intermittent, though, it'* hard to tell when the spray affects the idle. I did find, however, that the area around the bypass valve shaft leaks (and I noted that in a different post so others would know) and I found that there seemed to be a leak around the front driver'* side area of the supercharger base gasket; that'* why I removed the supercharger and replaced that gasket.
I've also inspected all vacuum hoses and fittings and replaced whatever looked suspicious, but the problem still remains. I've just about given up on trying to find a vacuum leak if there is one.
I've also suspected the fuel vapor canister purge valve or the fuel vapor canister as a possible cause, but I don't have spares of either of those parts to try swapping. I temporarily capped-off the vacuum line to the purge solenoid once, and I expected to see an immediate difference in idle quality or RPMs while it was capped, but it didn't seem to make a difference.

How well does your oil cap seal, and what'* the condition of your PCV and it'* o-ring?