Hyundai i30 FM radio channels change unexpectedly?

I am going to make a lot of assumptions in order to provide a plausible solution, the primary one being you are asking about FM channels.

FM receivers have a built in Automatic Frequency Control (AFC) function to keep the signal centered in the passband in order to maximize fidelity and ensure sub-carrier signals are received and processed. The AFC will be captured by the strongest signal in the passband causing the radio to retune itself as you describe.

The easiest thing for you to check is the condition of the components and connections of the receiver antenna system. While there could be a problem inside the radio, finding one there is much less likely than finding a broken, loose or disconnected antenna, a crimped or cut coaxial cable, or a damaged or loose antenna connector at the radio.

Good luck.

Hold in the mute button, it will remove the TP search