Unfortunately it is most likely either overload/intermod from other nearby transmitters.
400-500 MHz is a commonly used land-mobile band and it’s quite common to find repeaters and other transmitters which may push your LNA+RTLSDR into saturation.
It might be worth having a scan around 400-500 MHz (maybe using something like rtl_power, or GQRX) to see if you can find any obvious strong signals that may be giving you grief. If this is the case, then your only real solution is some kind of band-pass filter, possibly installed between the antenna and the LNA.
If you have another receiver, it would be good to set-up fully independent of the SDR experiencing the interference. If possible make portable with a battery powered set-up, so it’s not connected to the house wiring. This will allow roaming with the second receiver to do some direction-finding for the Radio Frequency Interference and confirm it’s not something with the power source allowing input through the house wiring.