In guided wave radar type flow meters we observe a sudden rise (100%) or a sudden dip(0%) in the level reading, even the process is stable.we are using magnetrol make.
I have had a similar problem with an E&H transmitter. The problem was the ferrule on the tank (the flange with its spool piece) was too long and this was interfering with the wave.
Refer to your instrument documentation - they will give you all the specs that you need for spacing etc.
First you check physically inspection of radar sensor. one important thing is there.GAP between sensor transmitter and receiver.if gap is more or distrb than it possible reading should be flucauting.