I have had similar problems with post-mont brakes in the past. MOST of the time, the probem has something to do with the top washer binding on the brake itself. Sometimes, it seems to be something with the bushing and the post. Regardless, if you choose to repace it (and I would if I did not feel like piddling around with it), there are two things to consider.

1). If you got to V-brakes, your levers will likely not work becasue the amount of pull required is different.

2). Just about anything is as good as or better than the standard LHT Tekro canti's (not intended to be a knock on them, they work and and don't cost a fortune). I have not tried the offerings from Paul/Rivendaell/Velo Orange or the latest Dia Comps, but I am sure they work fine. If you don't want to shell out the $$ for new/untested, then you ought to be able to find some mid-1980's Shimano's or Dia Comp's which will work virtually forever. If you have access to a bike coop, dig through what they have and I am sure you will finds some for little $$....ours had a box full of them.