92 losing rpm when accelerating

Help, my 92 3.0 litre is giving me grief. when i press the accelerator it loses rpm then it takes off a few seconds later. it also will stall when the pedal is pressed. my garage has checked many things and has had it for 5 weeks! they changed air filter, spark plugs, checked egr, changed coil pack with another used one and no change, new crank sensor, fuel filter, removed fuel tank and cleaned, i have cleaned what i believe to be the IAV(behind the oil filler cap?) with carb cleaner as suggested here on this forum. no change. it will idle with no problem and never stalls while just idling. it will move forward at idle speed but when the pedal is pressed, it conks out a bit and you have to "feather" the gas pedal to get it to go.
It all started one day when i drove 180 km's in the morning to go hiking. i drove it about 110- 130 km/hour and it was fine. then about 6 or 7 hours later on the way home this symptom of losing rpm was almost immediate. I do remember a rock jumping up under the car as i left the parking spot(down a mining road) and the garage looked underneath for any broken wires, hoses or whatever and found nothing. I really don't know what's next, i think i might take it to the dealer hoping that they will be familiar with this.