4 Wheel Drive/SUVs: 1990 GMC Sierra 305 Stalling, gmc sierra, rotten eggs

Question
Hi Matt, First let me say, I enjoyed reading all the questions and answers. You do deserve praise and thanks for all your knowlege, efforts and time involved. I definitely will tell friends about you!!
My dilemma is stalling and lack of power on my '90 GMC 305 4X4. My {Service Engine Soon} indicator keeps lighting up. I took it to schucks and had a computer reading done and he could not see anything wrong. Took it to Walkers muffler for another computer reading, he found the O2 sensor disconnected. Still stalling and lack of power. Have to feather the gas to keep from dying at stops and just to get it going down the road. Changed the catalytic conv. Had lots of power for 15 miles and the {SES} light came back on. Then lack of power results again and feathering to get it home. Drove it to town 2 days ago after setting for 2 days. Drove like new with lots of power. After getting gas to return home, {SES} back on with feathering gas to get back home. Bought new O2 sensor. Replaced with no better results. It runs good and then just changes an attitude to return. I am schedueled for a complete computer analysis Monday. I'm hoping for an answer from you before then that might help me before spending more money that I don't have. Thank You, Michael

Answer
Michael,

I'm not exactly sure what could be causing you problem right off, Does it smoke or smell rich out of the exhaust when it is running bad? Or does it smell like rotten eggs out of the pipes? I can help you better if you cn give me this information

Matt