rectify a scanned map

[Leonard Coop]

I have scanned a map of the western US (lambert projection)
and want to register it as lat-long.

i.rectify does a poor job.  It looks OK at the edges or in the middle
points but not both.  The error is as much as 200 miles.  The number of
points I use does not affect error very  much.  Is the problem
because i.rectify expects a more planar image (such
as a satellite image)?  Do I need a way to convert the lambert
(conical) projection into a cylindrical one?  Grass 4.0 does not seem to
have a module to convert coordinate systems in this way.  How do we
get a more accurate registration/rectification?

This must be a problem solved every day somewhere, but we are stumped for
now.