Epson 4490 Erratic negative scans??

Need a clue here. I tried scanning negatives and get mixed results. Sometimes it works great and right after a good scan I move the film strip (long film) and I get crap. I have tried everything and then eventually it might work for a while. It appears as tho the backlght isn't on, which it is and working. But something just isn't clicking. I've attached an example of a test.
I appreciate any help, Thanks, Fred_