Thanks to all who responded.  More news.  Yes, the check engine light is on
(it's been on for years).  Finally checked it today and the reader unit said
there was a problem with the cam position sensor.  This seems to make sense
since terrible misfiring seems to be the problem.

 Thing is, looking at symptoms for faulty cam position sensor don't seem to
cover my problem.  The things I have read seem to indicate there may be
problems starting and some lost power, but my problem goes way beyond that.

 My question is would an intermittently bad cam position sensor cause the
engine to lose almost all power, knock badly and smoke like a steam
locomotive when it goes bad?  Thanks.

And I don't mean going fast.  It recently started knocking really bad and
smoking like crazy.  It happens mostly as I start off, at a certain point it
will suddenly happen.  Then it might run ok for a while, then go back to
this knocking-smoking thing.  Sometimes I can hardly get going at all before
the knocking and smoking starts.  I have to really ease off the gas until I
get some momentum going.  Seems like whatever handles the timing just hangs
up.  I don't know.  Any ideas?  Thanks.