Welcome to the forum Jay!
At that speed, it's more likely that you are switching in and out of the overdrive.....There is a switch on the transmission called the TCC. It may be faulty. You should disconnect it and drive around to see if the switching stops. You may get lucky and find a bad connection or broken wire.http://www.kichline.com/chuck/fiero/TCC.htm
[This message has been edited by Gall757 (edited 01-15-2015).]