There are several problems that you can have with the shifter in the 90-99 model DSM transmissions. I’m not going to re-invent the wheel for this one being that Tom Stangl did a fine job explaining this stuff many years ago.
For all the shifter related into that you could possibly ask for (and more) please check here:
http://www.dsmtuners.com/forums/drivetrain-tech/93739-90-shift-linkage-vs-91-94-shift-linkage.html
Also make sure to read this page.