No former Board member or employee or DOT employee shall act as agent or attorney before DOT for anyone other than the United States in connection with any proceeding, application, request for a ruling or other determination, contract, claim, controversy, charge, accusation, or other particular matter, involving a specific party or parties, in which the United States is a party or has a direct and substantial interest and in which he or she participated personally and substantially through decision, approval, disapproval, recommendation, rendering of advice, investigation, or otherwise as a Board member or employee or DOT employee. (18 U.S.C. 207(a))