91 stalling please help

I have a 91 k1500 with a 350 it runs and idles fine in park if I put I put it in reverse or drive it idles rough but if I turn my ac on it stumbles even more and eventually stalls. I have codes for the egr and knock sensor. I replaced the egr but it still stalls I need any advice I am clueless

I have a 91 k1500 with a 350 it runs and idles fine in park if I put I put it in reverse or drive it idles rough but if I turn my ac on it stumbles even more and eventually stalls. I have codes for the egr and knock sensor. I replaced the egr but it still stalls I need any advice I am clueless

What are the code no?
Is engine holding good idle RPM or stalling from lose of RPM?
What is your engine vacuum readings?

Check post 3 in this link.

http://www.automotiveforums.com/vbulletin/showthread.php?p=6480495#post6480495