25C! The specs say -40C to +65C. Mine has worked happily in ~10C weather.
What does(n't) it do when it stops working?
They are very sensistive. Anything within about +/- 45 degrees (e.g. wires, landing gear) will be picked up. Mine can "see" a single wire fence from about 2m up.
The altitude just stays at 20cm. I tried to heat the sonar up with a hairdryer and it came to life once it got warmer. The daytime temperature in Singapore is around 32 deg C. It's in the mornings, that I notice this. It's fine when I am flying in the afternoon.
As you probably know, 20cm is the minimum reading.
Can you unmount it from the copter and point it in different directions to see if you can get normal readings?
Can you measure the voltage coming out of the sensor to see if it's changing?
Also check that your +5, ground and signal wiring is OK. I had the +5v wire break and the result was the same as you're seeing - stuck at 20 cm height.
Is it humidity? They are described as "indoor" so perhaps are sensitive to moisture.
I haven't been able to find specs on humidity but I have suspected that it doesn't work as well in wetter conditions (e.g. after rain).