Clutch is not adjusted properly.
You may have to have your clutch adjusted,
There is one cause for this. If the throttle body is moved or adjusted in any way this will cause the judder when engaging gears.
your clutch may not be fully engaging
BAD clutch slave cylinder, wore out clutch and pressure plate.
If the clutch fluid reservoir is just low, no. If the clutch fluid reservoir is empty, yes.
There are various ways through which you can know when your clutch is bad. When you are having troubles shifting gears, this might be a clear indication of a clutch problems.
get a new clutch
if there is a clutch there, you have to.
Could be that the clutch needs to be adjusted ... try double-clutching and see if the problem persists. Could be also that the clutch needs to be replaced.
Could be a bunch of stuff, but some of the earlier Fieros had an issue with the shaft connected to clutch pedal bending which would cause the clutch to not fully disengage. When your car's not running you can shift gears all day without pushing in the clutch. Try that when your fiero's running and all you'll do is chew gears.
Adjust your clutch cable.
your transmission is most likely bad if it an automatic but if it a manual then your clutch could be bad