Engine running or not? If it happens only with engine running, suspect clutch not fully disengaging (hydraulics, bent pedal, worn clutch shaft bushings, worn fork, etc.) If it happens engine running or not, could be shifter, cable, or possibly all of the above. Does it also happen in 3rd and 5th?
Sorry, probably only added to the confusion, but Fiero clutch systems are a pain. Every part in the system has to be in perfect working order.
1986 SE Aero coupe.
3.4 DOHC swap is complete and running, now just have to finish the rest of the car...