Sounds like you may want to invest in a new belt's.It could be the SC Belt slipping.So while you are replacing one,you might as well replace both belt's.
AC-Delco plugs 41-101 are about $38.00 for a set of 6 from local parts store.
AC-Delco plug wire set GM Part # 12192421 $36.70 shipped from
www.gmpartsdirect.com
Belt's you can pick-up at local parts store,either Gates or Goodyear Gatorback and both ACC and SC belt would run you around $60.00
So a ballpark figure of about 150 bucks for all those parts.
~Larry~