Got a 2001 b5 s4 with odd reading on volt guage. On start it reads 13-14 v like it should but drops to 12.5-12.30 after about 5 min. Batterey is new, belt not slipping. Can drop to 11.85 with max load. New alternator? Volt reg? Can the volt reg be replaced w/o dropping entire nose? Look forward to hearing thoughts EM