The v8s had a significantly different idle system from the early 4 and 6 cyl cars. The v8 idle controller's natural state is open. It has to be powered closed. It is powered by a variable duty cycle pulse. The more "pulse on" the more closed. It can be driven to 70% (70% is the highest duty cycle)closed. If it is 70% closed and idling too high false air (vacuum leaks) are occuring or the throttle isn't closed.

More commonly with symptoms such as yours the controller is deficient and releases control. This often is due to weak circuits occasionally repairable by resoldering.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician