Alan, I am almost postive I know what your problem is.  You have a bad ground
connection.  It could be either the ground strap on the driver's side head or
the ground strap on the tranny.  The oxygen light is coming on dim because the
alternator is not charging 100% because the ground is bad.  I had a broken
strap on my '85 and it messes the fuel injection system up so bad.  The van
ran like crap and got terrible fuel mileage (similar to what you are
experiencing).
Let me know what you find but I am almost postive this is what your problem
is.  If you need a new ground strap you can get a new, insulated one, from
your local Farm and Country store for $4.
Have fun.
Ken Wilford
Van-Again
John 3:16