Yes, the +/- lever is used for tabs. The + sets the tab and the - removes it. So you can actually see that the lever is setting and removing the tabs? 

Is so, then look at the tab linkage and brake stop. Unfortunately, I gifted my 2000 to another collector and have regretted it since ...  so I can't physically take a look. I've discovered that tab problems are mostly related to the brake just missing the tab stops.

Turn the machine around (remove the back plate) and look at the mechanisms while moving the carriage slowly. Something might be stuck or misaligned. Also check your margin release function as well as your right and left margin function. What I love about these machines is that you can usually figure what is failing by studying the mechanisms unless a part is missing or broken. Be slow and patient. And if you take anything off, take note or pictures of assembly and  keep screws in a container so you don't lose them. 

P.S. While you have the cover off, clean the area if you haven't done so already. 
 

I'm on the bottom of the expertise repair list. One with a 2000 will be able to explain it better and kindly post pictures. It's easier to see with pictures. Good luck and don't make the same mistake as I did by giving it away. IMO, they are fantastic machines.