The media sensor could be failing. I see a lot of my customers covering the sensor with tape due to end of media problems happening during printing. Make sure you didn't do that early on and forgot about it. If you are looking from the back of the machine the sensor is on the right side of the middle platen. It looks like a hole cut into the platen. Try using a flash light to get more light into the sensor to see if that works. If nothing else works, all signs are pointing towards the sensor being bad.