[cancritter]

l feel for ya...have you had it in to a GM dealer yet?
like yourself l dont mind payin the extra buck to have it done right the first time and if theres a prob the balls to stand behind thier work and tell ya straight up what prob is....
l realy dont know what to say..could be half a dozen things and for most here is like a doc doing a diagnosis over the phone...personaly ld take it to a reputable gm dealer and let them have a boo at it...at least youll have some sorta warentee in the end or a good idea whats wrong...
at this point have fergoten if you have internal engine mods to deal with as well...if so you gotta drag the whole thing to a dyno/mech shop or your just grabbin at straws trying to get it set up