An update if anyone cares
I Stoped the (what I thought was) thrashing! Thanks to the poster who said their fan broke....I opened the tower and watched the fans as the computer started going haywire, and realized there was an inch of crud all over the fan and the little grid thingy behind it (how's that for technical?!?!
) Took the fan out, cleaned the heck out of everything, and now the computer is quiet as a mouse.
SO, sorry you broke your fan, but thanks for the idea to check mine!