rpms idle gauge control

What is your question?

when driving in the fwy at 60-70mxh and i take the foot from speed pedal the rpm gauge slows downs and jumps a little while going down when then it keeps going down normal, i just spent $180 to replace intakes gaskets , mechanic pluged the scanner to check rpms and from 615 rpms jumped to 750 rpms and then when normal,anyideas plz