I would just take it back to who tuned it and get them to have a look, MIL came on so should be a fault code stored. A faulty AFM can cause cutting out, poor performance, and hunting at idle, but for it to swing between 1500-2000rpm, never seen that, sounds more like an idle stepper motor issue.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i