Hesitates during acceleration, occasional popping sound

Alright. At a loss here. While accelerating, my vehicle wants to hesitate. Has no power at all. If I floor it, it kicks in and drives normal. Problem normally occurs at speeds lower than forty mph. It is not all the time. Sometimes it runs really well for a couple days, other days it is barely drive able. On the bad days there is a distinct popping sound coming from under the hood. Sometimes while idling in park it will rev really high and almost stalls out. Here is what I've replaced so far:
TPS sensor.
IACV (idle air control valve).
New transmission fluid and filer less than 10,000 miles ago.
New distributor cap and rotor button.
Cleaned mass air flow sensor.
Fuel filter.
Thank you