High RPMs That Adjust In Time

[Amagician]

Howdy!

I have a 1996 Nissan Pathfinder and it's getting very poor mileage. I think the issue is that the RPM's run high and then drop to normal after some time driving (10 minutes or so). If I'm on the highway, hypothetically, and driving 65 mph, the RPMs will sit around 2,800. After 10 minutes or so it's almost like a gear is changed or something, as it will drop 300-500 RPMs and be fine. If I keep driving while the engine is warm, it's fine. It seems to be cold starts.

I let the car warm-up before I drive (don't put it in gear until it drops to 1,000 RPMs or less. At present, I get around 250 miles per tank.

I recently changed the TPS and adjusted it to 5v. I have, what I think would be considered, a stub air intake as the OE air intake ceased to be, and i recently had the distributor replaced. Could the stub air intake somehow be messing with the MAF?

I've searched this, and other, forums but haven't found a solid answer. Any help would be greatly appreciated.