I have seen the TACH give those exact problems. Its in the dash, as I presume it is the factory one.
Pull the instrument cluster and go drive it, if it clears up, then unhook the tach on the cluster panel.
If the issue is still there It could be a pickup coil in the dizzy, but I would lean towards the ECM. IIRC it has an EEC III type ignition, minimum function computer, and a module with a brown grommet. Somewhere in there is a governor,not sure if it is built in to the ECM or if it is another box under the dash.
I would start with the tach, as I have definitley seen those cause issues just as you describe.
Have a look inside the distributor, if there is no pickup coil, then it has the Duraspark III ignition system.Those have a crankshaft sensor in the back of the engine, not a pickup coil.
Fact is stranger than Fiction
Last edited by LATECH; 07-06-2012 at 07:54 AM.