The 2.5L engine on the '87 and '88 uses DIS (Distributorless Ignition System). The tach signal comes off the Ignition Module (white wire) and goes through the C500 then on to the tach. I'm betting it's just a bad tach.
Here is a link to various ECM pinouts that might help you in the future:
http://home.att.net/~subzero350/Also, a link to the 86 Factory Service Manual:
http://spad.sytes.net/fiero/manuals/Anyone have a link to the '88 FSM?
[This message has been edited by Hudini (edited 03-19-2008).]