why would my car jerk after the rpms fall below 1000 rpms

This description can have many answers. You may have a Driveability condition as simple as a Dirty Throttle Body, or worst case scenario, an internal transmission issue. If this only happens when the brakes are applied, you likely have excessive runout (warped) brake rotors. I would opt for the first since the Throttle Body is easily cleaned. The throttle plate and bore may become coated with contaminates from the intake system. This restricts Air flow into the engine when the throttle is closed. If the restriction is large enough, the Idle Air Control (IAC) Motor cannot compensate fast enough and a rough Idle or stalling will occur. PROCEDURE TO CLEAN THROTTLE BODY: 1. Remove the throttle body from the engine; 2. Spray the entire throttle body bore and both sides of the throttle plate with Mopar Throttle Body Cleaner, p/n 04897156AA. This will remove the bulk of the contaminant. CAUTION: This cleaner should only be used in a well ventilated area. Do not allow contact with skin or eyes. Rubber or butyl gloves and safety glasses are recommended. Wash thoroughly after use. Do not ingest. NOTE: Use only Mopar Throttle Body Cleaner, the use of other cleaning agents may damage components of the throttle body. 3. While holding the throttle open, use a small, (1” x 1”) piece of green Scotch Brite scuff pad, (or soft bristle brush), clean the throttle body bore and throttle plate, (including edges). NOTE: It is important that this cleaning procedure be performed thoroughly and completely. The edge of the throttle plate, and the portion of the throttle body bore where the throttle plate comes to rest are the most critical areas where the cleaning should be concentrated. These areas must be free of all deposits when the cleaning is completed. 4. Thoroughly clean the throttle body with Mopar Throttle Body Cleaner, p/n 04897156AA, and blow dry with compressed shop air. Inspect for any foreign material which may have been left from the cleaning process. CAUTION: Do not blow shop air pressure directly into the throttle shaft or throttle body housing holes. .