We've see that behaviour with a small number of new IR sensors, and recently I've changed the firmware to make it less likely to happen. But if the sensor has been working well for nearly 2 years and you haven't changed the bed surface, then the change might be caused by contamination of the optical components. Try cleaning them with a cotton bud moistened with isopropanol.
If you are in the UK then I can reprogram your sensor with the new firmware if you like. If you are outside the UK then the postage probably makes that uneconomical.