My mothers 1998 blazer with a 4.3 began shaking all of a sudden upon startup and then stalled. So considering the cars age and 197,000 miles I gave it a through tune up (plugs, wires, 02 sensors, iac and pcv valve, vacuum lines and a fuel pressure reg) if it was a sensor it was replaced. The car now starts fine and idles better but, the rpm's tend to surge. I took it to a garage and they told me it had low compression on 3 of the 6 cylinders. So i paid the ransom and brought it home. So I had done my own compression test and all cylinders were at 150 psi, (not a healthy psi but, hardly critical condition from what I read) am I missing something??

If the compression is the same in all cylinders that is a good sign. I am not really all that familiar with the spider injection but I think you have an IAC (idle air control) that is going bad.

Some fuel system cleaner in the tank would not hurt.

If that motor war O.K. before the problems and has that even comp. on all cylinders then 197k ain't really a bad thing .Check for a motor to chassis ground , broken/loose vacuum lines , Did you clean the iac ports ? You said it's idling better , but still not good ? ? You still don't know what the original problem was .

Thank you all for your responses and suggestions. I will be rechecking my work based on all your recommendations . I am glad to hear that the compression is fine.