Chiming in with a wild guess based on a quick look at the manual diagrams for this model...(I assume it is an 86)

There is a temp sensor, an EZL control unit, and a throttle position sensor in the system that all work together to advance and retard the timing in response to their various signal inputs. If you are certain the temp sensor is OK then I believe the throttle position sensor may lie at the root of your trouble. The temp sensor appears to advance the timing ,via vacuum, relative to temps above 95 degrees C, and the control unit appears to be controling things by moving the timing in response to temp and vacuum signals, so I think the throttle position sensor should be checked. Why?

It senses whether there is an idle position or throttle opened position. At idle the specified timing is between 3 & 7 degrees, and at throttle open the timing is between 24 and 28 degrees, the difference between these values being 21 degrees. These timing values are w/o vacuum but the difference in timing is the same when vacuum is present. I believe your throttle position sensor may be going out and incorrectly signaling the unit to advance the timing 21 degrees then retard it 21 degrees. It may be signaling the EZL to advance timing 21 degrees at idle and then signaling to retard it 21 degrees at throttle open.

If the throttle position sensor checks OK then I'd look at the EZL unit to see if it is mixing up signals or intermittently failing. The car may have been timed with the EZL unit having wrongly advanced the timing signal 21 degrees then it kicks out and the basic timing is then 21 degrees retarded.

I suspect one of these two units, and believe you should check the TP sensor first.

Might be worth looking into... just a thought.

Good hunting,

230/8