Voltage fluctuation likely caused by broken or missing Neutral connection at TNB substation.
For single phase residential houses, TNB will distribute the 3 phases evenly among the houses to achieve a statistically balanced loading. When loading on the 3 phases are balanced, a missing Neutral will not cause a voltage spike or a voltage dip. However, when the loading is unbalanced, the phase having a lighter load will see a increase in voltage and the phase having a heavier load will see a voltage dip.
Contact TNB to rectify the problem.