That is a very interesting problem. There could be many things that affect performance of the gyro and accelerometer sensors inside the IMU.
It could be external vibration from other parts of the robot that could be mechanically isolated or it could be an external noise source in your practice area that you could also isolate from.
It could be some sort of electrical interference or voltage spikes coming from either external source or a motor driver on the robot that is about to go bad.
However, it is very confusing that when you swap them out the newly inserted IMU starts to work immediately and the old one restarts to work the next day.
First, I would start suspecting your wire and ports where IMU connects to, which may be resulting in intermittent connection and erroneous data transmission. It is easy to check both wire and ports and connect with a new wire to a different port just in case. Would the error still show up right away if you disconnect all motor wires and disconnect and reconnect IMU wire only?
Then, I would look for any large electrical or noisy piece of equipment close to your practice area like an AC unit with large motor or a table grinder with a brushes that are about to go bad and throw a lot of sparks. Was IMU going crazy event coinciding with those motors running?
Also, I would get a thermometer with humidity meter and start logging both of those parameters every hour or so to see if this could be a possible environmental trigger for IMU failure event.
Finally, you could mechanically isolate your IMUs from any potential vibrations and wrap them in some sort of foam and foil to make sure there is no acoustic noise or EMI coming from external sources.