There are 4 adjustments, and they need to be done in the right sequence. The procedure is in the Service Bulletins, page 318-321 with photos. Its too much for me to re-write it all here. Les Andrews book spells it all out too. Your problem is correctable by using the slotted screw at the bottom, but the procedure for doing it requires that the worm play be properly adjusted beforehand. I have always done it on the bench after a re-build. The service bulletins say it must be out of the car, but maybe someone has invented a trick tool to access it in the car. The screw head is usually very stiff, and turning it requires a large screwdriver with a good blade and easy access.
As Don says, the cause could lie elsewhere too.