It's almost for certain the sensor strip. They are $225.00. But....before you do that, take it out and very, very carefully clean it with Isopropyl alcohol. Do not use denatured alcohol!
Clean it both sides with a really soft white cloth. Replace it. If it doesn't improve dramatically, then it's probably the sensor. It may simply be dusty. Blow it off with canned air.
Finally, the carriage cable has to be set to 11 lbs. tension. It's fourteen from the factory, but they wear in and finally settle at about 11. Ours was getting saggy, down to seven I think the tech said. That is probably not your problem. But if you're going to have a tech out, then definetly have him tension it. (takes special knowhow and tools, I think)
Hope you get 'er running pretty quick.