What result do you get when you use the "press to test" button on the RA? Depending on the model it would normally read 50' or 100' (IIRC), with a limited margin of error which would be found in the manufacturer's handbook.
If the PTT exceeds limits, that's a fair indication that the readout will be erroneous.
Then again, your pressure altimeter could be indicating up to 300' out, not the radalt.