MAZDA 626 1996 Fool or lower the revolutions

I own a 1996 Mazda 626 automatic. When I put it in "D" (Drive), it accelerates and then suddenly decelerates on its own, or the RPMs drop and then immediately rise again, stabilizing and continuing to do so. However, if I put it in "S" (Sport), it runs perfectly, but that's not ideal since its normal gear is "D" for smooth and complete shifting. I've been told it could be the MAF sensor, so I removed and cleaned it, but nothing changed. I want to be sure before replacing it since it's quite expensive. I've also changed the spark plug wires and spark plugs, but the problem persists.