I have a 95 Jimmy I just bought. I really love this truck but am having some issues with the idle and surging at cruising speeds. If I start it in the morning to warm up and let it run 10 minutes or so when I get in the SES is flashing which according to the manual that is a misfire code. I have had a constant SES and it read PO172 which is a system rich code bank one (I think) I have replaced the O2 sensor before the converter, basic tune up, cleaned the EGR still a very rough idle and erratic acceleration from stop. after reaching cruising speed seems to want to "surge". I don't really want to spend a bunch of money replacing needless parts to figure this out. I am leaning toward the cataylitic converter? The previous owner said they replaced the CPI spider with a used one...could it be that? I also read one of the threads where it wound up being a re-torqeing issue on the rockers any ideas on what route I should take first would be greatly appreciated

 

It sounds as though you may have a fuel leak in your intake. Possibly coming from the fuel pressure regulator on the used spider unit that was installed.
I believe the 95's are among the model years that tend to have the problem with the leaky fuel lines inside the intake. This is called a nut kit and should have been replaced along with the spider (as should the fuel pressure regulator)
If they weren't then it is a good bet that one or both may be bad and leaking fuel.

I would recommend running both a fuel pressure test as well as a fuel pressure leak down test first to rule out a fuel leak. Then you can go from there.

There are a multitude of threads here with tons of information on running these tests and what results are normal and what results will point to different parts being bad. As well as detailed information on how to replace those parts.

Let us know how it turns out.