Basically, this is a non-trivial problem, in anybody's book.
You have to build a bunch of new frames, from the current frames.
Fortunately, 1 out of every 6 frames is fine...you only have to
make the other 5.
You could just blend frames together, but that'd look pretty bad.
You really want to basically morph together two frames, warping
each one to fit the other, to make a composite that is crisp, but
is halfway between the two.
There is hardware to do this. There are machines called standards
converters, that do this; they are only OK, they add a weird shimmy
to the image; but they work in real time. Call any big post house,
it can't hurt to try this first.
There is software to do this. Kodak has a new tool called
Cinespeed that is designed exactly to solve your problem, it allows
the variation of speed of any sequence. Many PhD's spent many years
up in Rochester designing the motion analysis and morphing parts of
this program, it's amazing. Call Cinesite at 213-458-4000 and
ask if they'll do it for you as a service...it's probably cheaper
than buying the software.
I've written programs to do this too; but they require that the
animator specify the correspondences by hand...it's cheap, but it's
a significant amount of animator work.
Visual Effects Supervisor 408) 287-6770
Hammerhead Productions http://www.got.net/people/thad