Interesting fault. These clocks are the same internally as the XW/XY type. They work using a solenoid - contacts close, solenoid activates and flicks contacts apert, spring tension brings them back and the process starts again. The action is regulated by this rocking assembly (escutcheon) and a toothed wheel, so the spring tension is generally not a huge issue. Unless it has been stretched to buggery - this can occur through shortening it too much to tighten a spring which has lost its tension.
Just a thought.
Another option is to obtain the innards from a late model quartz analogue clock and retrofitting them in the case. Won't be able to pick it! Although the purists will spew, a lot of Jap cars may have what you need.
And dare I say it....and it chokes me to do this....um the VB
-VK commodore clock (bloody common) is only a little smaller.....may be a good innards donor.....