I have a Seth Thomas shelf clock that I recently purchased at an estate sale. When I first looked at it, it was running and was set to the correct time. I wandered around the estate sale for about half an hour and when I came back the clock was still running and on time so I bought it. Now that I have it at home it runs very erratically. Some days it keeps almost perfect time and on others it runs either fast or slow with no consistency. What's strange about it is that some nights I check it before I go to bed and the next morning it will be off by 6 hours; still running with the chime in sequence. I have also found it running but the hands are not moving. It's on my list of clocks to take apart. My question is what should I be looking for that would cause it to run like it does?