It's normal on the 1800's, when you hit the set button, then push the resume button for about 3 seconds and then let go, it will not drop speed then. There is a fix, it involves removal of the top shelter, The adjuster is located straight down the right side of the ignition key about 5 inches or so. You have to peel back the right plastic guard under the ignition to get to it. It helps to have a second person to hold the plastic back and shine a trouble light on the adjuster. It looks just like the throttle adjuster above the throttle, a long nut to adjust and a small nut to lock the long nut.
start the bike and run it until it comes off fast idle and is at normal idle speed, turn the long nut out until the RPM's start to increase, then turn it back in until it drops to normal, then tighten the two nuts together and your set.
Additional info. The lock nut is a 10MM and the long nut is an 8MM.