MGC

I have a 1998 LS400. A local auto parts dealer check for trouble codes and informed me that P1351 & P1354 codes show up as issues. The dash also lights up with a "check VSC" message. When idling at stop lights the engine runs rough but at higher speeds there is no sign of roughness. The car has 233,000 + miles. Can any help me on what the problem may be & how to fix it? Thanks in advance for your reply.