A faulty dimmer switch could produce the problem you describe, so possibly could a failing light switch.
However, a fault such as this needs to be remedied swiftly because it could be a potential fire hazard as the electricity is being restricted in some way which may produce over heating.
Has the wiring elsewhere in the house been altered just before the fault developed? Are there any other clues?
Frankly, this is a most unusual fault which may be difficult to trace without test equipment. Simply replacing parts is just guesswork and may work, but is more likely to result in spending money unnecessarily.
If you would like to find a reliable, insured and vetted tradesman in your area why not click through to the Find a Tradesman area of DIY Doctor: https://www.diydoctor.org.uk/find_tradesmen/ , complete the form and receive up to 5 FREE quotes
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!