Thanks for your reply. After blowing out the unit while closed, I opened up
the top cap. The speed control unit is sealed, as you said and sits on top
of the shaft. This makes me think that it is some sort of optical control
(LED controlled) as suggested by another poster. There didn't appear to be
any breaks in the seal, but iI blew it out where the shaft to the speed
control dial enters the unit. Was also able to blow out considerably more
dust with the top cap off than without (of course). All this and it still
jumps speeds. I have noticed that it only jumps to higher speeds though, it
doesn't get bogged down.
The mystery continues.
snip of an annoying problem.