When running a solenoid, curb idle is set at the solenoid. Idle stop screw (on the throttle body, solenoid disconnected) is then adjusted to prevent throttle plates from contacting throttle bores or up to a couple hundred rpms below curb idle to prevent dieseling when shutting down. A weak or problematic solenoid could collapse on sudden throttle closings creating similar symptoms you describe. If the engine returns to the original curb idle speed without throttle input from you after stopping/slowing likely not a solenoid problem.