Start out with a new servo to see if that clears out your problem. The electronics on the stock servo might be on their way out. Any dual bearing "standard" class servo will be fine.

For high torque on a budget: http://www3.towerhobbies.com/cgi-bin...&I=LXDTB3&P=ML

For a standard servo: http://www3.towerhobbies.com/cgi-bin...&I=LXVW07&P=ML

Either would be fine. the higher torque will give you more steering/holding power at higher speeds.