comint-previous-input mangles current line

[Joe Riel <joer@san.rr.com>]

Is there a simple fix for the following problem.
I'm running shell.  I then use C-up (comint-previous-input)
to scroll through the history of inputs.  This
mangles the input on the screen.  For example,
if I had

src $ echo nonsense
nonsense
src $ [type C-up]

The last line becomes

echo nonsensesrc $ 

That is, the previous command is displayed before the prompt.

Sometimes, when starting emacs this works fine.
Sometimes it does not.  When not, it never works
properly.  

-- 
Joe Riel