Bad grounds can cause all sorts of problems, from flickering lights to a coughing engine. Remember, it takes electricity to run the engine. Anything less than 12 volts can make an engine sluggish or tempermental or erratic. The lower the voltage, the worse the problem. Humid weather can make it perform better, because the moisture helps ground things. Hot dry weather can make it worse, because the grounds are poor.
For an electrical circuit to work as designed there needs to be good connections at every connecting point and no additional resistance added to the system. Poor grounds add resistance to the circuit, which lowers the voltage available to components that need it.
