You need to determine whether the drop is due to a fault in your building's wiring, or is due to a fault on the utility's wiring, probably outside your property. Unless you have the correct instruments, PPE and knowledge and experience (not likely if you are asking about mains supply issues here), measurements at the feed to your consumer unit should, for your own safety, be done by a licenced electrician.
Get a quote from a licensed electrician to check for excessive supply voltage variation, and check with the utility if they will charge you for the callout if no fault is found with their wiring, then decide who to book first! As previously mentioned, in rural areas fed by overhead lines, the odds are its a problem with the utilitiies wiring, but in urban areas its far more likely to be your wiring. In urban areas, as they try to keep the phases balanced by distributing properties on any street evenly between them, the odds are your immediate neighbours aren't on the same phase, but if you can, ask three neighbours either side if they are having similar problems - if anyone else is, it increases the probability that its the utility company at fault.