Have them give you a list of engine code errors which they will do free for you using an OBD device hooked to your car. That should give you some idea of what the problem is. Drop those codes here and we can look them up and get some ideas of what's wrong perhaps. RPM's flux while driving could be low fuel pump pressure from one that's failiing, or blockage in fuel lines (usually failing rubber hoses losing inside chunks that alcohol in fuel corrodes off them), or something ignition, or one of the sensors failing. The most common failure of sensor that causes that condition is the MAP or the manifold absolute pressure sensor. They have a thin metal film in them which measures the flex or stress and converts that to digital data which is then fed to the car's computer system. One way to check that is to disconnect the wires to that sensor and without hard driving, just slowly increases on speed, check to see if the fluxing of rpm's stops. If so, then that would definitely be an item needing replacing.
Some links for knowledge gain.
I took my car to a repair shop because I was having idling issues and rpms fluctuating while driving, they charged me $105 for a diagnostic fee then told me that I had several vacuum leaks, they ended up replacing vacuum hoses in engine compartment and vacuum lines to the intake runner assembly and charged me $524 including the diagnostic fee, and when i picked up the car I was having the exact same problems I was before and when i called back they said to drive it around more, which I did, and then he named something else and said that could possibly be the problem and said that the work they did was a "start" to findng the problem, and that i would need to bring the car back in for them to work on it more, and charge me again, wouldnt the diagnostic told him if there was another problem? I need any help i can get, Thanks