I do have multiple ports on my computer, but have always used the same one for the mic--incidentally, the one on the opposite side from the AC line... My laptop does have a metal case, which might be a contributing factor! It's just baffling to me that it's only an intermittent problem, grr. Not being able to reliably reproduce the problem makes it hard for me to feel confident I've eliminated it!

Thanks for the thoughts, & the tip on the high-pass filter. If it happens again, I'll give it a shot...