Well, I was finally able to get the PCV valve cover off. I used the two prongs of a wrench to push into the grooves of the PCV valve cover, and then I was able to get enough leverage to twist it counter-clockwise. It only cost $3 for the new PCV valve. The clerk at the auto parts shop said they're supposed to replace the PCV valve with every oil change. However, I doubt it's ever been replaced. I'm not sure what imaginary repair shop he's talking about when he says they should change the PCV valve with the oil change.
Anyway, I noticed the old PCV valve was coated in oil, and the ring had slid down halfway on the bottom part of the PCV valve. That may have happened when I removed it. I could shake the old PCV valve and it rattled, so I don't think it had stopped functioning altogether.
I installed the new PCV valve, and then started up the Buick. I hadn't started the car in over a week, so it sounded a bit choppy and unsteady at first. but then the engine smoothed out, and it sounded much better than the last time I started it. There still is an occasional lull or break in the engine idling, like a spark plug is missing. But it is improved.
However, it was a clear, dry, cool day in Austin, about 50 degrees. I'll have to try the Buick again when it's raining, because that's when the engine has responded horribly in the past. I think there's an improvement, but I'll give it another try soon and make sure. There still is something else going on though, and I suppose I'll check the coils, plugs, and wires next. But I'm optimistic that the new PCV valve actually improved the engine situation.
I'll post again soon when I've started it up again.