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