Put a timing light on it and watch the timing as the idle changes.... I'm betting the timing goes up & down directly in relation to the idle speed...
Looks like you have an MP electronic ignition? You have enough cam that in order to idle your on the edge of bringing in mechanical advance... And as soon as a couple degrees of timing come in the idle speed goes up... Then idle goes up more timing comes in... So the idle goes from 850-900 straight up to 2000+
The MP/DC electronic ignition is known for this...
Has any curve work been done to the distributor? Have you tried adding more timing to improve the vacuum?
generally more cam likes more initial timing, your engine probably would like 16-22 degrees of base timing... But if you crank in that much without limiting the total you'll hurt the engine....
I suggest you look into this...
http://4secondsflat.com/Mopar Mechanical timing limiter plate.htm
Plenty of us here could explain it but first use a timing light & verify thats your problem & if it is order the kit... The instructions are pretty good & will answer allot of your questions....
Proper timing setup is a big key to getting the most from your engine..
