The normal situation is the shift linkage gets tight. Then some clown
decides to free it up with a little heat, and melts out the plastic
bushings.

You need to rebuild the linkage - takes a set of plastic bushings, a
set of metal sleaves, and a set of new bolts and nylock nuts - plus an
hour or so of work.

This problem may indicate that the transmission linkage (the shifting arm
that you use to shift gears while driving)may be faulty. You might check
under the car in the area below the shift arm to check if there is any
free movement in the linkage hooked to the transmission (which is
connected to the shifting arm inside of the car). Sojmething may have
loosened, or worn to the extent that the linkage may need to be replaced.
Failing that there could be something wrong with the transmission itself.