My ThinkPad never had this problem before, but it now seems to only like the
upper slot.  If I put my mhzc in the bottom slot, the machine wedges.  If
it's at boot, it gets as far as

sm0 at mhzc0:
sm0:

or somewhere around there.

It's saying
sm0: invalid BSR 0x0000
sm0: unknown chip id 0, revision 0

even when it "works" (I'm just using it as a modem).

-s