[gr8shot]

the code 42 actually will probably be the main problem. That is the knock sensor. Pretty easy to replace yourself, or it should be pretty cheap for a shop to do too. In most cases the EGR shouldn't be causing that problem. the 35 shouldn't cause the problem your describing, but it could make it idle wierd, I'm not real sure about that one. The 54 code could also be your problem. It deals with the fuel pump, so there could be something wrong there.

here's the link to tell what the codes mean.

http://www.troublecodes.net/GM/93_577_tbi.shtml

Edit: I just saw that it doesn't show a code 35... wierd. That should be the IAC valve.