I had the semi-same problem with HTU21D. It returned with open circuit humidity and temperature a lot of times. After it worked again (or not :)).
The I2C line uses 3 eeproms which work fine all time...
I reviewed my code.. the library...the eeprom... bus frequency...datasheet... my 4 htu sensors, everything... the fault and instability stayed the same.
The final solution was: I used 1K resistors to pull up the bus lines because of 4 devices on it. This was the mistake. with 0.5K it also did not worked, but with 2.7K all of my sensors became available immediately.