However mint the 20CL might seem, it's still at least 25 years old. Even if the stylus is good, the suspension may not (or possibly will not) have the qualities it had when new.
Anyway, I would check the cables if all you did was sand off rust from the connectors. These turntables did not have RCA connectors originally, so the whole replacement may have been a horrible botch job to begin with, let alone now when everything has corroded.