what causes an engine to surge?

what are some causes of engine surge? i have an 02' GTS, and the engine began to surge on me. i had it pegged at a leaky vaccum line or the MAF sensor, but non of those seemed to be the problem. right now it is at the dealership and i had some heated words with the service rep. because they pretty much blamed me for the problem already, and they dont even know what is causing the engine to surge yet! they are blaming me because i was in there "playing"

i know it has to be an intake leak somewhere, but i couldnt find it! and it happened out of no where, i was just driving along and then all of a sudden the engine shot up to 4K rpm.. and now it bounces between 1.5-2K rpm constantly!

any ideas?

its at the dealer right now, but if anyone has had this and is sure of a fix i would be more than happy to pull it out or tell them what to do..