I can't give up on this. I've seen it before with my own eyes years ago and it kills me NOT to know the exact cause.

After an hour of Googling this evening, I found another case (open the page and search for "happy"). But the situation seems different than yours, and alas, no one in that thread responded to that poor fellow's call for help. Here's yet another case, but that thread wreaks of pure apathy toward researching the problem intelligently. It is interesting though that this problem seems to be linked to the Plus (as opposed to other compact models).

My gut tells me that someone on the logic board is non-stock. Could you post a crisp photo of the top of your Plus's logic board for us?

Also, have you tried other floppies?

I suppose you could pull the ROM chips, then carefully reseat them. Ditto for the RAM SIMMS (known good SIMMs, of course).