To repair a clock that is off beat, bend the verge one way or the other a small amount. Methods of repairing a clock vary based on the type of clock and the issue. If you can't solve the issue, take the clock to a repair shop.
The verge can bend when the clock is moved. As the only moving piece to touch the pendulum, the verge moves the pendulum back and forth. It's also responsible for the escapement of the clock, which generates the tick-tock sound. Most pendulum clocks that are off beat can be fixed by correcting the verge.
Clocks usually have a tool to adjust their time keeping ability when going too fast or too slow. For example, wall clocks typically have a pendulum bob on the bottom of the pendulum rod. This weight has a nut on the bottom. Turn the nut to the left to lower the bob, which slows the clock down. Turn the nut to the right to raise the bob if the clock needs to go faster.
If the clock is ticking but stops at times, the pendulum could be hitting something while swinging. This may be due to it having the wrong pendulum bob. It may also lack power in the time train. Clean the clock and add new oil if necessary.