RPM drop

I have the car listed above that at 30 MPH or above, even at 25 MPH at times, it will hit the average about 1500 MPH it needs to be at when trying to keep the speed the same then slowly as you apply the pedal more the rpms will go down and you can keep it at 1200 or 1300 or wherever within that 500 MPH drop and the car does not jerk or do anything its just losing rpms then when the pedal is applied far enough the car jumps up to 2000 rpm and will take off normal. Prior it did slip and jerk but it didn't at first then started to and got worse and eventually after a week blew my transmission. I paid 2400 had it completely rebuilt with top of the line parts and new lines and all. Got 30 minutes away from the shop and the MPHs did this dropping thing again but as I said now its with no other symptoms. There is no check engine light and as I said it drops slowly and gradually as you apply the pedal more until its applied enough to jump up to 2000 MPH and take off. I'm stuck between 3 things. MAF, IAC, or the pedal position sensor but I need help and opinions. I paid $3,000.00 for this car a month ago and within 3 week had to pay $2,400.00 for a transmission. This car drives amazing besides the MPH drop and I really need to get this figured out.