at times we get a 1/4Hz phase shifting of 2 sine waves and this causes a sort of dancing swing on the illumination. Not a perfect sync of grid. any further deviation circuit breakers trip at Substations and the faulty generating station would have to be taken off the Grid.
Locally, the Loose connections on the 230V/110V service lines crossing across lanes and sub lanes, get murky as the 11KV/410 or 230 Transformer is away from the customers house. In such cases , the only way is to call the electrical service people and show the problem, and coolly suggest without hurting their (dirty) ego, that the so called jumpers from the transformer to the customer's premises be cleaned and refurbished. this technique helped many times for me. Now luckily i stay in multi story building with Built-in Transformer.
Of course our other friends have already dealt with overloaded transformers.