I have a 92 firebird that had a broken TBI so I got one off ebay from a 92 camaro that went to carb, and now when I start the car the idle is where it should be but when I drive it around and stop at a stop light the idle gets stuck at 1000 rpm and sometimes 1500rpm. But when I start the car and take the rpm to 4000 and it let it go back to idle it goes back to the normal idle RPM. If I stop at a stop light and the idle is at 1500 I can turn off the car and start it back up the idle is back to where it should be.

 

Check to make sure that your throttle linkage is connected properly. If not, it can pull on the throttle a bit and open it up causing your high idle. After that, make sure that you do not have a vacuum leak at the base of the TBI unit. Did you use a new gasket?