To repeat, and maybe clarify what Bison is saying, if for whatever reason your turbo is running above/to the left of the surge line, you will get surge. Look at a compressor map. At a given air flow rate the compressor can only deliver a given pressure *ratio* before it starts to surge. If your engine is trying to take in some given amount of air, and the pressure ratio required of the compressor to deliver that air flow is above the surge line, then it will surge.
See the compressor curve I've attached. Suppose the air flow rate I picked is that required to cruise down the freeway. At point 1 the whole system - air filter, MAF, intercooler, throttle body, etc... - requires a low pressure ratio from the turbo to deliver that air flow. It doesn't surge. Now change up the MAF, the intercooler, something in the system, that makes the pressure ratio required go up. You still need that same air flow to get the car down the road, but now with the higher pressure ratio the compressor is above the surge line. And there you go - it surges.
Pressure ratio is the discharge pressure divided by the inlet pressure, measured right there at the compressor - P2 / P1 we call it. To get the pressure ratio down you either have to reduce P2, or increase P1. How do you increase P1, the suction pressure? Eliminate restriction - get rid of the MAF by going to speed density will result in a higher pressure at the compressor inlet. A bigger, freer flowing air filter will do the same. How do you decrease P2, the discharge pressure? Again, reduce restrictions. A lower pressure drop, freer flowing intercooler for example. I can see how changing intercooler, throttle body, upper intake, and piping might add enough pressure drop to move the system from one side of the surge line to the other. An intercooler half full of oil, or crapped up tubes, or a paper towel left in a pipe somewhere, etc... could add enough pressure drop to result in surging where there was none before.
John
