If you are talking about index downtube shifters, generally if they no longer hold the detent in index mode they are toast. You need to make sure they are in index and not somewhere in between friction and index. You can run them in friction until you decide what you want to do.

Depending on if they are 64xx or 65xx, basically 7 or 8/9 you can either find another module and swap it out, NOS/used lever replacement, or go to an STI.

The STI replacement will be Microshift, or some iteration of Claris or Sora. R2000/R3000 gets a bit funky with older FD’s.

John