I guess a vacuum leak could do this. We had an older C with a carbureted 460. Long time ago, but I remember the Cruise had to be supplied by Manifold Vacuum not Ported Vacuum. If the Cruise is using vacuum to control the Throttle and the Throttle is changing the Vacuum, you can guess it isn't going to go smoothly.
Also, our EGR valve failed, rusted out. When we replaced it, the system got out of whack again. I fixed that with an improvised "vacuum restrictor."
Looking at Amazon, they have several new cruise units around $200. 21st century electronic design.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB