[Donut]

I don't know if this would work better or not, but instead of using 3 line sensors and trying to keep the middle on the line, just use 2 sensors and always keep the line in between them. You can afford to be off a little bit more that way. I'm not entirely sure how well you can get a 3 line sensors system to work; my freshman year when I did autonomous for line tracking like this the best we ever got the robot to do was oscilate back and forth over the line while driving, though it never centered enough to drive straight.

What are you using to drive motors (command wise)? That could be the problem for twitching.

For the 3 sensors losing a signal, if the middle one loses the signal but the outer 2 have not been activated, then your line must be in between the middle and one of these 2 sensors, so you can continue straight. If one of the outer ones is activated and then loses it, I would continue to turn the way you were to fix it until the middle is activated again, indicating the line has been recentered. Otherwise you can't tell if you've lost the line outside of all the sensors or if it is in between the middle and outer ones.