There can be many reasons for your clock to keep stopping but below is a list of just some of the reasons that might be causing the problem.
#1 - Is your clock level?
Your clock needs to be 'in beat' to work properly. This just means that the 'tick' and the 'tock' are evenly spaced. If your clock has a pendulum then it is easy for the clock to get out of beat if say, the clock is moved.
Mantel clocks ideally need to be placed on a sturdy, level surface. Even if the clock worked fine on a surface that wasn't level but now doesn't work properly after being moved on to a level surface this is due to the clock being set in beat in it's initial (non-level) position so it will not be in beat on a level surface. The clock will need moving back to it's original position or putting back in beat to resolve the issue.
#2 - Is your clock secured?
Wall clocks and floor standing clocks need to be secured to a wall (ideally in a couple of locations for wall clocks) as the pendulums movement can cause the case to sway side-to-side (though maybe not perceptibly) which can eventually stop the clock. This is especially the case with weight driven clocks as when the pendulum bob and weights are at the same height the weights begin to swing, eventually hitting the pendulum causing it to stop swinging.
#3 - How long does your clock run for?
Most mechanical clocks are 8 day clocks, meaning they will run for 8 days on a full wind. If your clock doesn't run for the full duration then one of the above problems could be causing the stoppages or it is possible that the clock could be due a service. Services are recommended roughly every 10 years, whether or not the clock looks clean, as it's the parts that you can't see where the wear will be happening and the oil will be degrading.
While 8 day clocks are without doubt the most common, it's not an exclusive running duration, there are, for example, 30 hour clocks. So a good rule of thumb is to observe the running time between stoppages and make a note. If the stoppages seem to happen at the same point in a period of time, say, every time the clock reaches 11 o'clock, or if it stops randomly, maybe after 2 hours, maybe after 2 days, then it is likely that the clock will need an in-depth observation to diagnose the issue.