Since you mention 433 MHz, I assume you're in Europe. The 433 band is pretty smashed up from babyphones, remote controls etc. and is not well regulated.
If you have the option. I'd recommend that you switch to 868 MHz instead. The rules on power levels, and duty cycle are much stricter there. Mind you, you may run into conflicts with car key fobs, but as they're only active for a second or so, it's usually not an issue.