I suspect you may have a problem thats reasonably common on these
locks. If so it doesn't need a replacement lock, just repairing.
From an extremely vague memory there is a circuit board attached to
the back of the lock barrel and it works loose, giving intermitten key
operation. Its held in by metal tabls iirc, if you tighten the
mounting, it fixes the problem.

Note, the above may not be 100% accurate in the description, but if
you take a look at the think you should get the gist of what I mean,
and worth a try.