She failed to mention that she just blew a head gasket resulting in a valve job, complaining that they did an oil chance without authorization, So either she pissed them off and they put worn plugs back in it or they were like new. Kinda like the head bolts they didn't replace.
As someone who taught drivability you guys are speaking "alien" to her. The complete loss of power and correcting it's self by doing a wiggle test means that the wiring harness may have been damaged or has a loose connection due to breakage when serviced.
The 2 things that can cause this is the wiring controlling the coil packs either loose or damaged when they did the valve job or the fly by wire throttle body (TBA) actuator was left loose or it has bit the dust (see pic) as GM has had lots of issues with these and most shops replace the TB to cure it.
Codes for misfire or TBA would be helpful. Wouldn't be the first time a shop fixed something and either broke something or left it loose.
Get all codes in system written down, verify that the wiring and connectors look good(unplug and reconnect), CLEAN THE THROTTLE BODY WITH PROPER CLEANER AND TOWEL, clear the codes, and take it for a ride.
there are 2 actuators one on gas petal and one on TBA. TBA unit is about $250 for GM part, $175 for aftermarket or Amazon has a deal,
http://www.amazon.com/ACDelco-217-31.../dp/B001C6JUE8