I agree with 360beast, I'd be looking at the base setting for the throttle position switch and checking the accelerator cable adjustment.
I'd also check the wiring to the coolant temperature sensor, and I'd try unplugging the MAF to see if the problem continues with the MAF unplugged.
You don't mention a ragged/hunting idle, so I'd agree with your mechanic in that it doesn't sound like a vacuum issue to me.
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