The clock is out of beat,
its easy to resolve, the pendulium has to be totally level,
with age mantle clocks can wrap and expand and generally become unlevel.
get a level and adjust the clock (by sticking something under one of its legs) and see if the pendulium works now. this is a trail and error process.
if this works you know the clock is sound.
for a nicer job you need to adjust the pendulium.
simply remove the item you have under the clocks leg and put the clock where you want to keep it.
grab the pendulium arm and then slowly move it against the resistance to the left or right.
this will slowly adjust the "hang" of the pendulium.
then try running the clock.
eventually you should get the sweet spot and it should work perfectly.
its a trial and error process and takes patience
chris, September 2012
