Sounds like you may have one of the older DLE 20 engines that had the dreaded #4 ignition module. If you use a high idle setting as the engine's throttle is reduced to a certain speed, the module will dump all the ignition advance and the speed will just all of a sudden drop. This all happens at a speed I would call a "fast idle". Many people get around this by using a slightly rich LS needle setting but more users have found the only sure cure is to get rid of that ignition in favor of anything but the Rcexl #4. Maybe not your problem but you may want to look at your ignition module. If it has the #4 on it ..... I'll leave it to your own devices what to do with it. They only used that ignition module for a year or so then abandoned it in favor of one with a more normal advance curve.