Well there are quite a few things that casue a "surge" in a common rail diesel. You will need to check injector flow rates (scanner only). Fuel pressure regulator for pulsing (scanner only). Supply side fuel system for air leaks (seeing if it is sucking air into the system).
My bet would be the injectors, because you say the rpms aren't going up and down, which is usally what happens when the regulator goes bad, and when you **** air into the supply they have an extended crank. Report back with your findings.
If the engine surges, the RPMS will go up and down to the surge. If it only surges a little, you have to watch the RPM needle very closely and toy will see it move.
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I had the same idle surge problem. They replace the fuel pressure regulator and fuel pressure sensor. it fixed the problem. It cost me 1600.00 to fix because of the expensive OEM parts. Someone that's mechanically incline can do it themselves. Next time something happens, I will save me a bunch of money and do it myself.
Have someone with a decent scanner diagnose the problem. The should be able to tell you right away if you have injector problems. And do it soon because GM extended the warranty on the injectors out to 7 years or 200k miles. There is another test which shows up on the graph as a sawtooth pattern rather than a flat line which indicates a weak CP3 Injector Pump. The pump doesn't necessarily need to be changed when this occurs but it's a good idea if you can afford it. It causes the surging idle as well as some power loss and lower MPG.
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