ford 150

my 90 ford f150 had a tuff time smogging. The tech said he thinks the computer is malfuncitioning. When i sit at stop light the motor reves down then back up. I had it tuned up and it still did it and then one day it stopped. My battery died the other day got a jump and now it is doing it agian. It smells like it my be running to rich? easy fix? drive it til it stops?